Lithium-sulfur batteries represent a promising candidate for advanced rechargeable batteries with high energy density and long cycling lifetime. Although extensive efforts have been devoted to the research and development of mediators for sulfur cathodes to enhance its redox kinetics, their working mechanism is still a mystery. Further advancement in lithium-sulfur batteries requires a more fundamental understanding of its electrochemical properties and processes. We report herein some mechanistic insights into the electronic structures of active materials and various mediators (e.g., metal oxides, metal sulfides, metal carbides), as well as their interactions. This work offers not only an effective strategy to achieve fast redox kinetics in lithium-sulfur batteries, but also guidelines for future material design.