Role of Electron-Deficient Imidazoles in Ion Transport and Conductivity in Solid-State Polymer Electrolytes

Abstract

Solid-state polymer electrolytes offer a safer alternative to traditional lithium-ion batteries based on organic electrolytes. The focus is here on imidazole(Im)-functionalized polymer electrolytes, where the Im ligand promotes salt dissolution, while its functionalization allows tuning the dynamic interactions between the cations in solution and the Im ligand tethered to the polymer backbone. Although careful choice of a polymer backbone and Im linker functionality have resulted in polymer electrolytes with increased total ionic conductivities and a boost in the Li+ ion contribution, improvements in performance through modifications of the Im heterocycle remain underexplored. In this work, we systematically investigate poly(methylsiloxane) polymers functionalized with a series of halogen-substituted imidazoles and show that Li+ ion conduction can be tuned by an electron-deficient heterocycle ligand. When the number of halogen substituents increases, the Li+ ion mobility also increases as measured by pulsed-field-gradient nuclear magnetic resonance (NMR) and NMR relaxometry. Although beneficial for Li+ transport, electron-deficient Im ligands result in clustering, as indicated by wide-angle X-ray scattering, and poor salt dissolution, which in turn impedes the overall ionic transport. This work highlights the importance of synthetic design and the necessity for high salt solvation and weak cation–polymer binding to obtain high Li+ transport numbers.