Metal/Graphene Composites with Strong Metal-S Bondings for Sulfur Immobilization in Li-S Batteries

Selections of metallic cathode materials and modulations of metal-sulfur bonding strength are crucial for sulfur immobilization in development of high-performance lithium-sulfur (Li-S) batteries with low cost. By combining theoretical calculations and experiments, herein we reveal the relationship between intrinsic electronic structure and metal-S bonding strength, which links to energy density and durability of Li-S batteries. Through first-principles calculations, we simulate sulfur clusters (S-1, S-2, S-4, and S-8) immobilization on metal (Cu, Ni, and Sn) slab surfaces with and without graphene substrate. For sulfur clusters, the metal-S-x (x = 1, 2, 4, and 8) bonding strength is in the sequence of Ni > Cu > Sn without graphene substrate. Nevertheless, the sequence changes (Ni > Sn > Cu) in the presence of graphene substrate due to different amounts of charge transfer between these metal clusters and graphene. Guided by these theoretical results, metal (Cu, Ni, Sn)/graphene (G) composites are synthesized and subsequently integrated into the cathode of Li-S batteries. Among these metal/G systems, the sulfur cathode with Ni/G composites demonstrates remarkable electrochemical performance, i.e., a discharge capacity of >830 mAh g(-1) over 500 cycles with an average Coulombic efficiency close to 100%. These findings shed light on theoretical calculations providing insights into the electrode design of Li-S batteries.