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Summary: Room temperature sodium-sulfur (Na-S) batteries hold great promise as the next-generation cost-effective energy storage systems. However, their practical implementation is still plagued by the low reversible capacity of a bulk-sized commercial sulfur cathode with low Coulombic efficiency and poor cycling stability. Here, we report a highly stable room temperature Na-S battery using a facile-processed, nanocarbon-promoted, bulk-sized commercial sulfur cathode and a polymer electrolyte. This processed nanocarbon possesses a high binding affinity to sulfur and polysulfides, largely facilitating the sulfur reaction kinetics and leading to high reversible capacity. Meanwhile, by applying a thin coating of the facile-processed nanocarbon on the polymer electrolyte, dead sulfur formation can be avoided, contributing to greatly enhanced capacity retention. Our Na-S battery delivers a reversible capacity of >700 mAh g−1 with near-100% Coulombic efficiency and the ultrahigh capacity retention of 98.2% at 0.2C after 200 cycles.