Preparation of multi-wall carbon nanotube/S composites as cathodes for lithium/sulfur batteries

Multi-wall carbon nanotubes (MWCNTs) with different diameters were ball-milled with different amounts of elemental sulfur for 2 h under an argon atmosphere, and then heated at 155℃ for 24h followed by heating at 300℃ for 2h to prepare MWCNT/S composites. The composites were characterized by N2 adsorption, scanning and transmission electron microscopy, and electrochemical tests. It was found that the composite showing the best performance was the one in which the MWCNT diameter was in the range 10-20nm and the sulfur content was 85 mass%. SEM confirms that the composite has a MWCNT-core and sulfur-shell structure. Sulfur not only covered the surface of the MWCNTs, but also filled the voids between the tubes. For the optimized composite the initial discharge capacity is 1272.8mAh · g-1-sulfur and the capacity remaining after 80 cycles is 720.1mAh · g-1-sulfur. Its active material utilization and capacity retention rates are 76.0 and 64.4%, respectively. The active material utilization and cycling performance of the composite are greatly improved compared with a sulfur electrode without the MWCNTs, owing to a high electrical conductivity of the MWCNTs and the stable structure of the composite.