Sulfur encapsulation by MOF-derived CoS₂ embedded in carbon hosts for high-performance Li-S batteries

Li-S batteries have attracted great attention for their combined advantages of potentially high energy density and low cost. To tackle the capacity fade from polysulfide dissolution, we have developed a confinement approach by in situ encapsulating sulfur with a MOF-derived CoS₂ in a carbon framework (S/Z-CoS₂), which in turn was derived from a sulfur/ZIF-67 composite (S/ZIF-67) via heat treatment. The formation of CoS₂ was confirmed by X-ray absorption spectroscopy (XAS) and its microstructure and chemical composition were examined through cryogenic scanning/transmission electron microscopy (Cryo-S/TEM) imaging with energy dispersive spectroscopy (EDX). Quantitative EDX suggests that sulfur resides inside the cages, rather than externally. S/hollow ZIF-67-derived CoS₂ (S/H-CoS₂) was rationally designed to serve as a control material to explore the efficiency of such hollow structures. Cryo-STEM-EDX mapping indicates that the majority of sulfur in S/H-CoS₂ stays outside of the host, despite its high void volumetric fraction of ∼85%. The S/Z-CoS₂ composite exhibited highly improved battery performance, when compared to both S/ZIF-67 and S/H-CoS₂, due to both the efficient physical confinement of sulfur inside the host and strong chemical interactions between CoS₂ and sulfur/polysulfides. Electrochemical kinetics investigations revealed that the CoS₂ could serve as an electrocatalyst to accelerate the redox reactions. The composite could provide an areal capacity of 2.2 mA h cm^-2 after 150 cycles at 0.2C and 1.5 mA h cm^-2 at 1C. This novel material provides valuable insights for further development of high-energy, high-rate and long-life Li-S batteries.