UC Santa Barbara

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 and are thus detrimental to performance. 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 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 ionic/electronic 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, while the structure of the side chain largely governs electron transport. These fundamental insights are then bridged into real battery binders, showing that an electrostatically stabilized complex, comprising of a blend of a charged conjugated polymer with an oppositely charged polyelectrolyte, reduces kinetic limitations in LiFePO4 cathodes. The conducting binder dramatically improves both rate capability and cycle stability, compared to the industry standard, insulating PVDF binder. Finally, the method of electrostatically stabilizing conjugated polymer complexes is shown to be an effective platform for mixed conducting binders, as several polymer chemistries afford high-performing, conductive binders.