Lithium-sulfur (Li-S) batteries are widely recognized as the most promising next generation energy storage system owing to the incredibly high theoretical energy density and specific capacity. Nevertheless, the practical application of Li-S batteries is hindered by the inevitable “shuttle effect” of the soluble lithium polysulfide, which leads to fast capacity degradation, low coulombic efficiency and severe self-discharge. To address these issues, we propose an ultralight carbon flakes modified separator (CFs@PP) that improves the conductivity and works as an effective polysulfide barrier for high-performance Li-S batteries. The CFs are easily obtained by the direct carbonization of sodium citrate, making it possible for large scale manufacture. With the CFs@PP separator employed, the Li-S battery delivers an initial discharge capacity of 1063 mAh g−1 and retains 683 mAh g−1 after 500 cycles at 0.5C with a low capacity decay rate of 0.071% per cycle. Moreover, great rate performance and small self-discharge are also acquired with the CFs@PP separator applied. The facile preparation of the CFs and the superior electrochemical performance of the CFs@PP separator may promote the commercialization of Li-S batteries.