Molecular Origins of Polymer-Coupled Helical Motion of Ions in a Crystalline Polymer Electrolyte

Direct observation of migration pathways of ions and a quantitative dissection of their energetics in solid polymer electrolytes (SPEs) are essential to understand the molecular origins of barriers limiting the conductivity of these novel materials. Depending upon the interplay between molecular packing and dynamics, SPEs exhibit a wide range of conductivity (10(-9)-10(-4) S/cm) at room temperature despite their common polymer matrix. Detailed molecular studies are needed to establish a precise correlation between the nature of polymer packing, dynamics, energetics, and ion conduction for rational design of SPE-based fast ion conductors. In the present article, a novel method is developed to observe directly the polymer-coupled transport pathways and associated energetics of ions in a crystalline SPE (PEO3:NaI). The anions follow a distinct helical path intertwined with the polymer helix to form a double-helix-like tunnel that facilitates the migration of the cations within it. The tight molecular packing and the presence of long-range correlations in the crystal facilitate collective hopping of ions with a strong coordination between the cation and anion transport. The abrupt changes in the conformation, helical pitch, and radius of gyration of the polymer accompanying ion hopping indicate a strong coupling between polymer dynamics and ion transport and reveal important clues about transport-promoting dynamical modes of polymers and associated structural changes in the crystalline SPE. The calculated free energy profiles provide accurate estimates of the activation barriers for the cation and anion transport in crystalline PEO3:NaI.