Structure and mechanisms underlying ion transport in ternary polymer electrolytes containing ionic liquids

Abstract

We use all atom molecular dynamics simulations to investigate the influence of 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF₆) ionic liquid on the structure and transport properties of poly(ethylene oxide) (PEO) polymer electrolytes doped with LiPF₆ salt. We observe enhanced diffusivities of the Li⁺, PF₆^-, and BMIM⁺ ions with increasing loading of the ionic liquid. Interplay between the different ion-ion and ion-polymer interactions is seen to lead to a destabilization of the Li-PF₆ coordination and increase in the strength of association between the Li⁺ cations and the polymer backbone. As a consequence, the polymer segmental relaxation times are shown to be only moderately affected by the addition of ionic liquids. The ionic-liquid induced changes in the mobilities of Li⁺ ions are seen to be correlated to polymer segmental relaxation times. However, the mobilities of BMIM⁺ ions are seen to be more strongly correlated to the BMIM-PF₆ ion-pair relaxation times.