The theoretical energy density of 2.5 kWh/kg of sulfur, plus its low-cost availability and environmentally friendliness, makes of lithium-sulfur (Li-S) batteries one of the most promising alternatives to fulfill the need of energy storage devices for high energy-demanding applications such as electric vehicles and smart grids. However, the successful commercialization of Li-S batteries is still challenged by the low utilization of sulfur, linked to its poor electronic/ionic conductivity and its continuous irreversible dissolution into the electrolyte in the form of long-chain lithium-polysulfides. So far, the mixing of sulfur with carbon has led to its better physical confinement and chemical sequestration in the cathode. However, the details of the sulfur-carbon interaction and its effects on the electrochemical lithiation processes during battery cycling are still largely unknown.

Here we aimed at characterizing the structure of sulfur/graphene-nanosheets composites (SC composite) and understand its effects on the electrochemical lithiation processes during battery cycling. Reactive molecular dynamics (ReaxFF) calculations provided information on the cathode microstructure and the bulk reduction products upon lithiation. Ab initio molecular dynamics simulations (AIMD) along with density functional theory (DFT) optimizations, yield details of the effects of different electrolyte compositions on the SC composite capabilities for retention of sulfur and the polysulfide formation paths.

We elucidated that a stronger sulfur-carbon interaction not only contributes to a better electronic conductivity but also leads to an alternative sulfur reduction path without the formation of long-chain polysulfides, which might help to explain the improved cycling stability observed in sulfurized polyacrylonitrile composites reported in the literature. In addition, we observed that different electrolyte compositions might be beneficial too by allowing an easier recovery of S-C bonds upon charging. We believe the results from this work can contribute to improving the synthesis methodologies of sulfur-carbon composites leading to a more rapid commercialization of lithium-sulfur batteries.