Abstract

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. This work elucidates fundamental polymer design strategies for simultaneous lithium-electron conduction, while also considering the practical requirements of a battery binder (i.e. processability, electrochemical stability, and solubility). First, it is shown that side chain engineering can be used to control lithium transport in semiconducting polymers, emphasizing the importance of solvating ions without trapping them. This concept is further explored, studying Li + transport and electron conduction in a family of polythiophenes functionalized with cationic side chains. It is found that the interaction strength between the side chain and added salt is critical for ion transport, and tuning this interaction strength largely governs Li + mobility over that of its counterion. The structure of the side chain predominately influences electron transport in these conjugated polyelectrolytes. These fundamental insights are then applied to battery binders, showing that electrostatically stabilized complexes, comprising of a blend of a charged conjugated polymer with an oppositely charged polyelectrolyte, reduce kinetic limitations in LiFePO 4 cathodes. Notably, complexation overcomes inherent dissolution issues associated with the single component conjugated polyelectrolytes, while also enhancing electronic conductivity compared to the single component polymers. These properties are demonstrated with a variety of polymer chemistries, where the conducting binders dramatically improve both rate capability and cycle stability, compared to the industry standard, insulating PVDF binder. Ultimately, the method of electrostatically stabilizing conjugated polymer complexes is shown to be an effective platform for balancing ion and electron conduction in organic polymers, with particular utility as high-performance battery binders.