 Ionic liquid (1-ethyl-3-methylimidazolium tetrafluoroborate) treated carbon nanotube (CNT) sponges were tested as a conductive matrix and polysulfide reservoir for the cathode of lithium–sulfur batteries. X-ray photoelectron spectroscopy results confirmed that this treatment doped fluorine and oxygen on the surface of the CNT, and experimental results showed that this treatment had significantly improved adsorption of polysulfides in the CNT sponge. As a result, this sponge cathode accommodated a remarkably high sulfur areal loading of 8 mg cm−2, showing a high areal capacity of 7.1 mAh cm−2 at the 100th cycle at an areal current density of 1.28 mA cm−2 with an average capacity fading of 0.048% per cycle. The adsorbing energy of Li2S6 on the F/O-doped carbon structure was calculated using the density functional theory, confirming that the doping made the polysulfide adsorption stable particularly due to fluorine. This study provides a useful approach of simultaneously introducing both fluorine and oxygen to carbon in order to significantly improve the polysulfide adsorption on the carbon cathode and thereby obtain high areal discharge capacity, which is much more important than specific discharge capacity for actual battery operation.