Morphology and Ion Transport in Zwitterionic Polysiloxanes.

Abstract

The recent discovery of superionic conductivity in doped polyzwitterions (PZIs) has opened a new direction for polymer electrolytes (PEs). Solid-state battery research is currently dominated by inorganic electrolytes due to their rigidity and high conductivity, but they suffer from poor electrode adhesion and processability issues. While PEs overcome these issues, they typically suffer from low ionic conductivity. Recently, salt-doped PZIs have emerged as promising candidates, where high concentrations of ionic species and local dipolar interactions can reduce energy barriers for ion transport and therefore increase room-temperature ionic conductivity. Here, we demonstrate that superionic conductivity is not a universal behavior of doped PZIs, but rather a strategic design of ion-transport pathways is required. Using a flexible polysiloxane-based PZI doped with LiTFSI salt, we observe that room-temperature conductivity remains limited (∼10-8 S/cm at 293 K) despite a relatively low glass transition temperature and high dielectric constant. X-ray scattering reveals the structural origin of this behavior; namely, zwitterionic groups aggregate into isolated nanodomains separated by the polymer matrix, which slows down ion mobility. Once these clusters start overlapping, doped PZI showed significant enhancement in conductivity. These results demonstrate the critical role of percolating ion channels for superionic conductivity in PZIs.