Cobalt sulfide catalysts (CoS and CoS₂) with different phases, electrical conductivities, porosities, and specific surface areas are synthesized and loaded on a carbon paper (CP) interlayer which has a role to support an electrically conductive network for maximizing the catalytic activity of cobalt sulfide. Based on the detailed diagnosis about electrochemical results, cobalt sulfides reveal their electrocatalytic activity to promote both polysulfide reduction and Li₂S nucleation during discharge. We propose a new interpretation about the capacity vs. voltage profiles of Li–S cells, which can elaborate on the reduction reaction of sulfur during the discharge reaction. Through the electrochemical examination, we determine the role of cobalt sulfide catalysts as well as CP interlayer and the critical factors of cobalt sulfides for the enhanced performances of Li–S batteries. Our work provides new insights for understanding about the catalytic activity of cobalt sulfides and designing advanced catalysts for the high utilization of sulfur in Li–S batteries.

High energy ball-milled iron sulfides with thin carbon layer coating (BM-FeS/C composites) were prepared by the simple and economical process. Ball-milled process, followed by carbon coating, reduced the particle size and increased the electrical conductivity. When employed as sodium-ion battery anodes, BM-FeS/C composites showed extremely high electrochemical performance with reversible specific capacity of 589.8 mAh·g^-1 after 100 cycles at a current density of 100 mA·g^-1. They also exhibited superior rate capabilities of 375.9 mAh·g^-1 even at 3.2 A·g^-1 and 423.6 mAh·g^-1 at 1.5 A·g^-1. X-ray absorption near edge structure analysis confirmed the electrochemical pathway for conversion reaction of BM-FeS/C composites.

Iron sulfides have been considered as one of the most promising candidates for sodium ion battery anode materials due to their high theoretical capacity and low cost. In this work, spindle-like Fe₇S₈ with nitrogen-doped carbon (Fe₇S₈/N-C) nanohybrids are successfully synthesized via a solvothermal method by sulfidation iron-based metal organic framework (FeMOF). As sodium ion battery anodes, Fe₇S₈/N-C nanohybrids exhibit high reversible capacity of 450.8 mAh·g^-1 at 200 mA·g^-1, and 406.7 mAh·g^-1 at 500 mA·g^-1 even after 500 cycles. They also show excellent rate properties and delivering the capacity of 327.8 mAh·g^-1 at a very high current density of 3.2 A·g^-1. These outstanding electrochemical performances can be attributed to the unique structure of Fe₇S₈/N-C nanohybrids. The nanoscale dimension in their size can be beneficial for facile ion and electron transports. Furthermore, the stable nitrogen doped carbon frameworks can also improve electrical conductivity and relieve the problems related to volume expansion. X-ray absorption spectroscopy and X-ray photoelectron spectroscopy analyses have been performed to study reactions occurred in spindle-like Fe₇S₈/N-C nanohybrid electrode at both bulk and surface.