The thermal decomposition behavior of multi-walled carbon nanotubes (MWCNTs) was revisited to prepare partially opened MWCNTs by a simple air oxidation method which allows mass-productivity. As a promising matrix for sulfur cathodes, the partially opened MWCNTs can incorporate sulfur in hidden internal areas in the carbon nanotubes, leading to a homogeneous distribution through the MWCNT matrix. The resulting materials enable Li–S batteries to exhibit greatly enhanced electrochemical performance.