To Hop or Not to Hop: Unveiling Different Modes of Ion Transport in Solid Polymer Electrolytes through Molecular Dynamics Simulations

Although the basic modes of ion transport in solid polymer electrolytes (SPEs) are already classified and well-described, their distribution in typical polymer electrolytes is not clear and neither are the effects on the distribution by different degrees of ion-ion and ion-polymer interactions. Here, the ion-transport mechanisms in poly(ethylene oxide) are studied along with poly(ε-caprolactone) at different molecular weights and LiTFSI salt concentrations using molecular dynamics simulations. Through tracking of the cation coordination changes, three transport mechanisms are categorized, i.e., ion hopping, continuous motion (successive exchange of the coordination sphere), and vehicular transport. The observed dominant transport mechanism is in all cases continuous motion, which changes from polymer-mediated to anion-mediated with increasing salt concentration, while polymer-mediated vehicular transport is not observed to be a major contributor to cation transport. In both systems, ion hopping is also essentially absent, as can be expected in systems with strong ion-polymer interactions. The results illustrate how the usual description of ion transport in polymer electrolytes as coupled to segmental motions is too simplistic to catch the full essence of the ion-transport phenomena, whereas the frequently used notion of "ion hopping" in the majority of cases is incorrect for SPEs.