Sulfur-deficient MoS2-x promoted lithium polysulfides conversion in lithium-sulfur battery: A first-principles study

The lithium-sulfur batteries are becoming the promising next generation large-scale electrical energy storage, due to the high specific energy density and theoretical capacity. During the charge/discharge cycle, the shuttle effect reduces the Coulombic efficiency and cycle performance, greatly limiting the practical application of lithium-sulfur batteries. Two-dimensional materials metal sulfides are increasingly used in lithium-sulfur battery cathode materials due to their low lithiation voltage, good conductivity, strong adsorption and catalytic effects of lithium polysulfide. Recently, sulfur-deficient MoS2-x was creatively introduced into the cathode material, leading to better electrochemical performance than the perfect MoS2-x. The modification factor of sulfur deficiencies for metal sulfides is an innovative approach to further suppress the shuttle effect and improve the lithium-sulfur batteries performance. Understanding the specific working principle of sulfur-deficient MoS2-x would help to expand the application of defect metal sulfides. From experiment, the sulfur deficiencies are involved in the polysulfide conversion and enhance the conversion kinetics. However, the intuitive picture of how to enhance the conversion kinetics and the specific origins for the promotion effect are still unclear. A systematic theoretical study has been carried out on the molybdenum disulfide involved discharge process from S-8 to Li2S. According to the calculation results, both the discharge process on MoS2 (001) and MoS2-x, (001) surface undergo three consecutive lithiation-cleavage mechanism, with the last cleavage step Li4S2@MoS2 -> Li2S@MoS2 as the rate-determining step. For MoS2-x (001), the product Li2S@MoS2-x is thermodynamically more stable, and the energy demands for the three cleavage steps are less than that of MoS2 (001) surface, which provide more thermodynamics and kinetic driving force for polysulfides conversion. Originally, the sulfur deficiency results in larger charge densities on MoS2-x (001) surface, enhance the interaction with the product and monomer molecules after cleavage, finally promotes the corresponding thermodynamic and kinetic processes.