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

Abstract

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 similar to 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.