Adiabatic and Nonadiabatic Charge Transport in Li–S Batteries

Abstract

The insulating nature of the redox end members in Li–S batteries, α-S and Li2S, has the potential to limit the capacity and efficiency of this emerging energy storage system. Nevertheless, the mechanisms responsible for ionic and electronic transport in these materials remain a matter of debate. The present study clarifies these mechanisms—in both the adiabatic and nonadiabatic charge transfer regimes—by employing a combination of hybrid-functional-based and constrained density functional theory calculations. Charge transfer in Li2S is predicted to be adiabatic, and thus is well-described by conventional DFT methodologies. In sulfur, however, transitions between S8 rings are nonadiabatic. In this case, conventional DFT overestimates charge transfer rates by up to 2 orders of magnitude. Delocalized holes, and to a lesser extent, localized electron polarons, are predicted to be the most mobile electronic charge carriers in α-S; in Li2S, hole polarons dominate. Although all carriers exhibit extremely low equ...