Mechanistic Insights into Interactions of Polysulfides at VS Interfaces in Na-S Batteries: A DFT Study.

Room temperature sodium-sulfur (Na-S) batteries, because of their high theoretical energy density and low cost, are considered as a promising candidate for next-generation energy storage devices. However, the practical utilization of the Na-S batteries is greatly hindered by various deleterious factors such as dissolution of sodium polysulfides (NaS) into the electrolyte commonly termed as "shuttle effect," sluggish decomposition of solid NaS, and poor electronic conductivity of sulfur. To overcome the challenges, we introduced single-layer vanadium disulfide (VS) as an anchoring material (AM) to immobilize higher-order polysulfides from the dissolution and also to accelerate the otherwise sluggish kinetics of insoluble short-chain polysulfides. We employ density functional theory (DFT) calculations to elucidate the NaS interactions at the VS interfaces. We show that the adsorption strengths of various NaS species on the VS basal plane are adequate (1.21-4.3 eV) to suppress the shuttle effect, and the structure of NaS are maintained without any decomposition, which is necessary to mitigate capacity fading. The calculated projected density of states (PDOS) reveals that the metallic character of the pristine VS is retained even after NaS adsorption. The calculated Gibbs free energy of each elementary sulfur reduction reaction indicates a significant decrement in the free energy barrier due to the catalytic activity of the VS surface. Furthermore, VS is found to be an excellent catalyst to significantly reduce the oxidative decomposition barrier of NaS, which facilitates accelerated electrode kinetics and higher utilization of sulfur. Overall, VS with strong adsorption behavior, enhanced electronic conductivity, and improved oxidative decomposition kinetics of polysulfides can be considered as an effective AM to prevent the shuttle effect and to improve the performance of Na-S batteries.