Structural Dependence of the Sulfur Reduction Mechanism in Carbon-Based Cathodes for Lithium-Sulfur Batteries
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2017 American Chemical Society. Lithium-sulfur batteries constitute a promising energy storage technology due to their high theoretical specific capacity and energy density. However, their commercialization has been hindered due to the low cycling life of the devices caused by long chain polysulfide shuttling between electrodes, the low conductivity of sulfur, and the volumetric expansion upon sulfur reduction. Nanostructured porous carbon cathodes have been the object of extensive research due to their proven capacity to overcome the aforementioned issues, though there is still a limited understanding of the physical-chemical behavior leading to high electrochemical performances. Herein, quantum chemical calculations are used to study the effect of the structure of porous carbon cathodes on the sulfur reactivity in lithium-sulfur batteries. Ab initio molecular dynamics (AIMD) simulations are initially employed to evaluate sulfur reduction mechanisms and kinetics in a confined environment. A porous cathode architecture is modeled through parallel graphene layers with elemental sulfur rings in the interlayer and filled with 1,3-dioxolane (DOL) organic solvent and lithium ions. AIMD simulations showed fast reduction of elemental sulfur and formation of short chain polysulfide. Carbon dangling bonds at the cathode edge were confirmed to enhance the reactivity through strong sulfur-carbon and lithium-carbon interactions. Furthermore, adsorption calculations through density functional theory (DFT) proved the capacity of small pores to retain long polysulfide chains, providing a complete atomistic description of the physical-chemical behavior of sulfur and polysulfide in porous cathodes of lithium-sulfur batteries. Finally, this work also demonstrates the influence of the specific current on the amount of sulfur utilization and practical specific capacity of the battery.