(109a) Conjugated Polymer Coacervates as Batter Binders

Polymeric battery binders are a ubiquitous component in composite lithium-ion cathodes, providing critical structural functionality. However, industry standard binders, such as polyvinylidene fluoride (PVDF) are insulating to both electrons and ions, detrimentally adding resistance to the overall system. Mixed ion-electron conducting polymers are promising materials for next generation battery binders, as they can provide the adhesive properties of traditional binders while also facilitating charge transport. However, simultaneously optimizing electronic, ionic, and lithium transport within a single system has proved a challenge, particularly given the need to maintain the mechanical function required of a binder. Further the practical requirements of a battery binder are extreme including: high solids-loading processability, electrochemical stability, and resistance to solvation in the battery electrolyte. In this talk, I will show that complexation between two oppositely charged polyelectrolytes offers unique opportunities for solvent-lean processing of semiconducting polymers, facilitated by the formation of a polymer-dense fluid phase known as the coacervate. These electrostatically stabilized complexes, comprising of a blend of a charged conjugated polymer with an oppositely charged polyelectrolyte, reduce kinetic limitations in LiFePO4 cathodes. Further, complexation overcomes inherent dissolution issues associated with the single component conjugated polyelectrolytes, while also enhancing electronic conductivity compared to the single component polymers. Across a wide variety of specific polymer chemistries, these conducting binders dramatically improve both rate capability and cycle stability, compared to the industry standard, insulating PVDF binder. Further, via manipulation of electrostatic parameters, including polymer charge fraction and counterion concentration, we can adjust the morphology of these polymer complexes from a homogeneously mixed state to a weakly structured state in which the local ordering arises from backbone-immiscibility-induced segregation. Our findings demonstrate that structural disorder along the CPE backbone is alleviated in strongly mixed complexes due to the nanoconfinement induced via the charge complexation. As a result, charge transport is improved in charge complexed materials in comparison to the analogous conjugated polymers.