Linear and Cross-Linked Ionic Liquid Polymers as Binders in Lithium-Sulfur Batteries

A collection of different polymeric ionic liquids (PILs) were explored as cathode binders in lithium-sulfur batteries. The PIL molecular structure, polymer backbone, and polymer architecture were found to influence the cell capacity, the cyclability, and the morphology of the cathode itself. PILs with styrene backbones performed better than the vinyl-based polymer, while cross-linked PILs imparted further improved capacities, cyclability, and reduced overpotentials. Unlike polyvinylidene fluoride, PIL binders mixed with the sulfide species, resulting in more uniformly distributed sulfides in the cathode and better sulfide transport. These features helped to mitigate volume change-induced degradation that typically plagues Li-S batteries. The uptake of polysulfides by PILs also constrains the polysulfide shuttle during battery cycling, leading to better cycling stability. While traditionally binders are viewed only as a glue to hold the active material together, PIL binders have additional functions and play an active role during Li-S battery working operation.