Li+ Transport Mechanism in Oligo(Ethylene Oxide)s Compared to Carbonates

Molecular dynamics simulations have been performed on oligo(ethylene oxide)s of various molecular weights doped with the lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI) in order to explore the mechanism of Li+ transport in materials covering the range from liquid electrolytes to prototypes for high molecular weight poly(ethylene oxide)-based polymer electrolytes. Good agreement between MD simulations and experiments is observed for the conductivity of electrolytes as a function of molecular weight. Unlike Li+ transport in liquid ethylene carbonate (EC) that comes from approximately equal contributions of vehicular Li+ motion (motion together with solvent) and Li+ diffusion by solvent exchange, Li+ transport in oligoethers was found to occur predominantly by vehicular motion. The slow solvent exchange of Li+ in oligo(ethylene oxide)s highlights why high molecular weight amorphous polymer electrolytes with oligo(ethylene oxide)s solvating groups suffer from poor Li+ transport. Ion complexation and correlation of cation and anion motion is examined for oligoethers and compared with that in EC.