Importance of the Ion-Pair Lifetime in Polymer Electrolytes: From all-atom MD simulations

Ion pairing is commonly considered as a culprit for the reduced ionic conductivity in polymer electrolyte systems. However, this simple thermodynamic picture should not be taken literally, as ion-pairing is a dynamical phenomenon. Here we construct model poly(ethylene oxide)−bis(trifluoromethane)sulfonimide lithium salt systems with different degrees of ion-pairing by tuning the solvent polarity and examining the relationship between the cation−anion distinct conductivity σ+−d and the lifetime of ion pairs τ+− using molecular dynamics simulations. It is found that there exist two distinct regimes where σ+− d scales with 1/τ+− and τ+−, respectively, and the latter is a signature of longer-lived ion pairs that contribute negatively to the total ionic conductivity. This suggests that ion pairs are kinetically different depending on the solvent polarity, which renders the ion-pair lifetime highly important when discussing its effect on ion transport in polymer electrolyte systems.