Computational Study of Electronic Current Effect on Sulfur Conversion Reactions Under Lean Electrolyte Conditions
Authors: Soto Fernando Antonio , Hankins Kie , Balbuena Perla B. 
Published: February 2020

The increased demand of portable electronic devices, transportation applications, and for the seamless integration of renewable energy sources (e.g., solar and wind) into the grid has prompted the research efforts towards the development of high energy density batteries [1]. In this respect, lithium-sulfur batteries (LSBs) have attracted the attention of the scientific community. Li-S batteries have a high theoretical specific capacity of 1673 mAh/g, and an energy density of energy of 2500 Wh/kg, which makes it an ideal material for the electrochemical energy storage system [2].

However, LSBs show a reduced life cycle mainly due to the dissolution of insoluble polysulfide chains (e.g., LiS2 and Li2S2) in the electrolyte. Besides the so-called “redox shuttle reactions,” where the polysulfide species diffuse to the anode prompting the dendrite growth [3], these polysulfide chains also precipitate on the cathode surface forming a passivating layer that makes the electrode electrochemically inaccessible [4]. Past efforts to retain polysulfide chains include nanostructure carbon/sulfur, graphene oxide/sulfur, carbon nanotube/sulfur and polymer/sulfur composite materials as trapping networks to prevent polysulfide dissolution into the electrolyte [5-7]. More recently, studies show that the amount of electrolyte needs to be reduced to an electrolyte to sulfur (E/S) ratio less than 5 mlE/gS to achieve the desired high energy density [8]. Despite the experimental research efforts, a complete fundamental understanding of the effect of lean electrolyte conditions on sulfur reduction mechanisms is still lacking.

This work investigates the sulfur reduction mechanisms in a graphene/solvent environment using ab-initio molecular dynamics (AIMD) simulations. Here, several current rates were tested in an environment where the dioxolane (DOL)/sulfur ratio was set to less than 10 mlE/gS to investigate the role of rate current and electrolyte/sulfur ratio in reduction mechanisms and sulfur utilization. We analyze S speciation and actual lithiation compared to the theoretical values as a function of cathode capacity. We also analyzed the Li trajectories to elucidate any impedance effects exerted by the solvent molecules. A molecular-level understanding of these reactions and trajectories is expected to enable an atomic-scale design of LSBs.

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