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.