  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 PEO-LiTFSI systems with different degree of
ion-pairing by tuning solvent polarity, and examine the relation between the
cation-anion distinct conductivity $\sigma^\rm{d}_{+-}$ and the lifetime of
ion-pairs $\tau_{+-}$ using molecular dynamics simulations. It is found that
there exist two distinct regimes where $\sigma^\rm{d}_{+-}$ scales with
$1/\tau_{+-}$ and $\tau_{+-}$ respectively, and the latter is a signature of
longer-lived ion-pairs which 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.

Soft Condensed Matter;Chemical Physics