Fabrication processes of fossil fuel-derived carbon nanomaterials are of high carbon emissions. Deriving carbon materials from low-cost and sustainable biomass is eco-friendly. Cotton, one of the most abundant biomass materials, naturally holds a hierarchically porous structure, making the activated cotton textile (ACT) an ideal scaffold for loading active materials. Here, we report a low-cost approach to massively producing multiwalled carbon nanotubes (MWCNTs) via a combination process of vapor–liquid–solid (VLS) and solid–liquid–solid (SLS) where cotton decomposed into carbon-containing gases and amorphous carbons that then dissolved into Fe nanoparticles, forming Fe/Fe3C-encapsulated MWCNTs. The lithium–sulfur (Li–S) battery constructed by the Fe/Fe3C-MWCNT@ACT/S composite (as the cathode) and the Fe/Fe3C-MWCNT@ACT (as the interlayer) exhibited a superlative cycling stability (over 1000 cycles at 1.0 C), an ultralow capacity decay rate (0.0496% per cycle) and a remarkable specific capacity (1273 mAh g–1 at 0.1 C). The Fe/Fe3C-MWCNTs enhanced electrode stability and suppressed polysulfide dissolution during cycling.