While solid polymer electrolytes (SPEs) possess a combination of the requisite properties to merit their use in allsolid-state rechargeable batteries, their low ionic conductivity (sigma) at room temperature limits their widespread use in advanced commercial applications. To overcome this limitation, it is necessary to understand the crucial molecular factors that govern the energetics and transport of ions in SPEs. In this Article, all-atom molecular dynamics (MD) and adaptive biasing force (ABF) simulations are used to characterize the mechanism and energetics of polymer-coupled ion transport and to probe the nature of interactions of polymers and ions in a crystalline SPE (PEO3:NaI). In particular, the free energy profiles associated with the cation transport along the polymer helix, the activation energy for hopping of a cation between adjacent energetically favourable coordination sites, and crucial factors determining the features of the free energy profiles are examined. The calculated free energy profiles exhibit well-defined minima (correspond to favourable cation coordination sites) separated by relatively high energy barriers (12-14 kcal/mol) indicating that ion hopping is the likely mechanism of cation transport in PEO3:NaI crystals. The influence of the conformational flexibility of the polymer on the energetics of cation transport is also examined. The dissection of interaction of the cations provides insights into the interplay of ion-ion, ion-polymer, and polymer-polymer interactions in determining the activation barrier for cation transport in the crystalline PEO3:NaI.