Facile Synthesis of CoS Nanoparticles Anchored on the Surface of Functionalized Multiwalled Carbon Nanotubes as Cathode Materials for Advanced Li–S Batteries

The effects of CoS loading (10, 20, and 30%) on multiwalled carbon nanotubes (MWCNTs) on electrochemical performance for Li–S batteries were investigated. A facile hydrothermal method was used to grow CoS nanoparticles in situ on the surface of MWCNTs. In addition, CoS on functionalized multiwalled carbon nanotubes (f-MWCNTs) was also investigated. Scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy were used to characterize the structure and electronic state of CoS on the surface of MWCNTs and f-MWCNTs. The initial discharge capacities and reversible specific capacities after 200 cycles were found to be 980 and 612 mAh/g, 990 and 707 mAh/g, and 959 and 658 mAh/g for the 10, 20, and 30% CoS samples, respectively. Interestingly, the 20% CoS@MWCNTs show the best initial discharge capacity and retention after 200 cycles. This finding can be attributed to the fact that high CoS loading (30%) may not be dispersed evenly on the MWCNT surface, while low CoS loading (10%) leads to fewer CoS and weaker adsorption for polysulfides. Moreover, the 20% CoS/f-MWCNTs show even higher initial discharge capacities of 1103 mAh/g and reversible specific capacities of 796 mAh/g after 200 cycles. This can be attributed to a rougher surface with more active sites on f-MWCNTs for trapping polysulfides and for enhancing the electronic conductivity of the cathode composite. X-ray photoelectron spectroscopy (XPS) spectra show prominent peaks at 162.7 and 163.9 eV, which correspond to Co–S and C–S bonds, respectively, suggesting that CoS bonds well with MWCNTs. It is postulated that CoS/f-MWCNTs play a role in adsorbing lithium polysulfides (LPS) during the charge and discharge process and accelerating the kinetics of polysulfide conversion.