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 ∼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.