Combined Nudged-Elastic-Band and Constrained DFT calculations of ion transport at an electrified interface

Electrified interfaces present challenges for the modeling of electronic structure under periodic boundary conditions with standard approaches to defining the electron density via orbital occupation. The assumption of a common, homogeneous Fermi level can lead to charge transfer events that do not respect the necessary time scales of electron transfer, particularly non-metallic interphases. This is problematic when attempting to determine kinetic barriers for charge transport through such interphases in the presence of potential differences. Using constrained density functional theory (CDFT) we can maintain electronic structure in metastable states while exploring atomic and molecular configurations during charge transport. We report the methodological combination of the nudged-elastic-band approach, for determination of kinetic barriers between distinct, stable atomic configurations, with CDFT to explore lithium ion transport through sulfur towards a negatively charged graphene electrode. Our results indicate metastable local minima and well-defined barriers which would limit ion transport and delay the chemical conversion of sulfur to polysulfides via electronic reduction.