Carbon nanotubes are chemically activated to create hierarchical multiscale porosity while maintaining an excellent electrical conductivity. The lithium-sulfur battery consisting of an activated carbon nanotube (aCNT)/sulfur cathode and an aCNT-modified separator with an optimal composition delivers an excellent reversible capacity of 621 mAh g−1 after 500 cycles at 0.5 C with an exceptionally low degradation rate of 0.043% per cycle. The fascinating performance arises from the ameliorating functional characteristics of aCNTs in both the two battery components. The highly porous, conductive aCNT network facilitates fast electron/ion transport and easy electrolyte penetration into the cathode, facilitating full utilization of active materials. The aCNT layer deposited on separator effectively constrains the polysulfide shuttling by physically blocking their migration to the anode while immobilizing polysulfides by strong adsorption onto the micropores of aCNT surface, as validated by microscopic images and molecular dynamic simulations.