Li+ Transport in Single-Ion Conducting Side-Chain Polymer Electrolytes with Nanoscale Self-Assembly of Ordered Ionic Domains

Abstract

Li-ion batteries based on organic liquid electrolytes have been commercialized for decades. However, the flammability of the liquid electrolyte and propensity for reaction with metallic lithium anodes warrants the study of alternative electrolyte materials to satisfy modern demands such as higher safety and energy density. Polymer electrolytes are less flammable than organic liquids, more easily processable than inorganic solids, and more inert toward lithium metal and electrochemical side reactions. But the ionic conductivity in common polymer electrolytes, such as poly(ethylene oxide)-based polymer electrolytes, is limited by the segmental relaxation of the polymer matrix that is solvating the lithium cation. Recently, metal ion-containing polymers with regulated, repeating chain architecture have drawn attention as ion conductors due to their ionic domain segregation. In this contribution, we investigate the ion transport mechanism in a series of metal ion-containing polymers with ionic groups located on the side chains to explore their potential as Li+ conductors. Four side-chain polymers having different numbers (n = 6, 10, 12, and 15) of methylene groups as side spacers between the polymer backbone and terminally bound anions, titrated with Li+ counterions, were synthesized and characterized. These polymers were found to have strong nanoscale phase segregation with predominantly 1-D ionic domains. Through dielectric spectroscopy analysis, their conductivity was found to be linearly scaled with the dielectric relaxation rate. Short side chains (n = 6) resulted in a slower dielectric relaxation rate and lower DC conductivity compared to polymers with longer side chains (n ≥ 10). The long-range Li+ transport in these polymers is found to be coupled to the ionic cluster relaxations.