Lithium-sulfur (Li//S) batteries constitute promising high energy storage devices but suffer from serious Li polysulfide (LiPS) shuttle effect and sluggish redox conversion. Herein, we report that these challenges can be tackled successfully by introducing oxygen defects in W₁₈O₄₉via Cu doping. The oxygen defects in Cu-dopedW₁₈O₄₉ (Cu/W₁₈O₄₉) are critical not only for enhancing the chemosorption toward LiPS but also for promoting the electrocatalytic conversion of LiPS. A reaction pathway for electrocatalytic conversion of LiPS to Li₂S₂/Li₂S is proposed based on various analyses. As a result, the Cu/W₁₈O₄₉ modified PP (Cu/W₁₈O₄₉@PP) separator with a thin composite layer of 2.3 um and 0.25 mg cm^-2 displays high ionic conductivity, fast Li+ ion transport and super-electrolyte-philicity. The Li//S battery with the Cu/W₁₈O₄₉@PP separator shows superior cycling stability with a low capacity decay rate of 0.046% per cycle at 1 C over 300 cycles, up to 5 C discharge rate, and ultralow self-discharge behavior. Our work provides fresh insights into the design of electrocatalysts for fast redox conversion in advanced Li//S batteries.