Revisiting the HKUST-1/S Composite as an Electrode for Li-S Batteries: Inherent Problems That Hinder Its Performance

Abstract

A composite formed by the HKUST-1 metal-organic framework (MOF) and sulfur (S) was studied as an electrode for Li-S batteries. For preparation, a simple milling procedure in ethanol, to facilitate its homogeneity, was used instead of melt diffusion. Different instrumental techniques were used to characterize the structural, morphological and textural properties of the compound. The results revealed a MOF with a high degree of purity, a high specific surface area and a dual micropore and mesopore system. Although capacity retention is good, losing 0.88 mAh g−1 per cycle, capacity values are low, 200 mAh g−1 at 100 cycles recorded at C/10 rate. Furthermore, the electrode only gives a moderate electrochemical performance at intermediate current rates. In parallel, a review was carried out of the reported results of this MOF for this application. Although better performances of the HKUST-1 MOF have been published for this application, in some cases, the lack of experimental data to confirm them and serious doubts about the actual content of S in the composite bring the results into question. Only by increasing the additive content, working with low S loading, increasing the measurement voltage window and lowering the current rate can the electrode deliver higher capacities. The inherent problems of this MOF and others with similar properties are indeed their insulating character since the textural properties are adequate for S impregnation and absorption of the polysulfides formed.