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Tremendous effort has been put into developing viable lithium sulfur batteries, due to their high specific energy and relatively low cost. Despite recent progress in addressing the various problems of sulfur cathodes, lithium sulfur batteries still exhibit significant capacity decay over cycling. Herein, we identify a new capacity fading mechanism of the sulfur cathodes, relating to Li_xS detachment from the carbon surface during the discharge process. This observation is confirmed by ex-situ transmission electron microscopy study and first-principles calculations. We demonstrate that this capacity fading mechanism can be overcome by introducing amphiphilic polymers to modify the carbon surface, rendering strong interactions between the nonpolar carbon and the polar Li_xS clusters. The modified sulfur cathode show excellent cycling performance with specific capacity close to 1180 mAh/g at C/5 current rate. Capacity …