Polarizability of poly(ethylene oxide) enhances cooperative coupling between Li+ and segmental motion

Ion transport in polymer-based electrolytes is governed by coupled ion-polymer dynamics, yet the role of polymer polarizability remains unresolved. While molecular dynamics simulations have provided insights into the transport mechanism, simulation studies have often neglected explicit polarizability due to a high computational cost, especially for polymeric systems that require long time trajectories to estimate the transport properties. In this study, we consider lithium hexafluorophosphate (LiPF6) in poly(ethylene oxide) (PEO) melts and perform all-atom molecular dynamics simulations using the APPLE&P force field. We turn on and off the polarization term in the APPLE&P Hamiltonian as a perturbation test and investigate the effects of the polarization term systematically. We find that the polarization term of PEO chains makes the Li+ ion bind strongly to the oxygen atoms of the PEO chains. Such strong binding of the Li+ ion to oxygen atoms slows down the hopping of the Li+ ion between neighboring oxygen atoms of the PEO chains but facilitates the cooperative motion of the Li+ ion with particular PEO chain segments at intermediate time scales. The enhanced cooperativity plays a key role in the ion conductivity and the self-diffusion of the Li+ ions in PEO melts.