LOCAL-STRUCTURE AND MOBILITY OF IONS IN POLYMER ELECTROLYTES - A MOLECULAR-DYNAMICS SIMULATION STUDY OF THE AMORPHOUS PEO(X)NAI SYSTEM

Solid polymer electrolytes are ionically conducting phases formed by d issolving salts in an amorphous polymer matrix. In this study, the loc al structure and dynamics of Na+ and I- ions in molecular dynamics (MD ) simulations of the amorphous poly(ethylene oxide)-based electrolyte PEO(x)NaI (x=48,20,3) are analyzed at both 400 and 500 K. The fully at omistic model reproduces many phenomena seen experimentally and provid es a picture of the complex correlations between cation, anion, and po lymer host in these systems. The composition of the first coordination shell around the cations illustrates the concentration-dependent comp etition between iodines and PEO backbone oxygen atoms to coordinate th e positively charged cations. Contiguous polymer segments tend to form near-planar polydentate loops around the sodiums while the anions are usually placed above and/or below the PEO ... Na+ quasiplane. This ge ometry results in optimal coordination of both types of ligands to the cation in a similar pattern to that found in crystalline PEG-based sy stems and crown ethers. Although the observation of large-scale diffus ion is still effectively precluded by the currently accessible MD time scale, discrete ligand exchanges do occur in the coordination shell o f the cations. Examples of these dynamical events, including various j umps between local potential coordination sites, are shown and their n et result on ionic motion is discussed. (C) 1996 American Institute of Physics.