Lithium-sulfur (Li-S) batteries suffer from rapid capacity decay and low energy efficiency because of the low solubility of lithium sulfide (Li2S) in organic solvents and the intrinsic polysulfide shuttle phenomenon. Here, a novel additive, phosphorus pentasulfide (P2S5) in organic electrolyte, is reported to boost the cycling performance of Li-S batteries. The function of the additive is two-fold: 1) P2S5 promotes the dissolution of Li2S and alleviates the loss of capacity caused by the precipitation of Li2S and 2) P2S5 passivates the surface of lithium metal and therefore eliminates the polysulfide shuttle phenomenon. A Li-S test cell demonstrates a high reversible capacity of 9001350 mAh g1 and a high coulombic efficiency of 90% for at least 40 stable cycles at 0.1 C.