Rechargeable lithium–sulfur (Li–S) batteries are recognized as one of the most promising candidates for next-generation energy storage devices due to their high energy density and high theoretical capacity, along with abundant natural resources, low cost, and the non-toxicity of sulfur. However, applications of Li–S battery have been hindered by limited specific capacity due to the poor conductivity and low content of sulfur, as well as fast capacity fading caused by polysulfide shuttling problems. Here, silver iodide is introduced as a host material for fabricating sulfur cathodes for Li–S batteries. Silver metal and lithium iodide, as the discharge products of silver iodide, can improve the redox environment, increase the ionic and electronic conductivity, and inhibit the shuttle of polysulfides during the charge and discharge processes. With a high sulfur content of 90.4%, the S/AgI composite delivers enhanced cycle performance with a decay rate of 0.092% per cycle within 500 cycles at 0.5 C, thanks to the synergistic effect of silver and lithium iodide. This opens the door to the rational design of halogen-containing transition-metal compounds as a class of cathode materials for Li–S battery.