Sulfidation of iron confined in nitrogen-doped carbon nanotubes to prepare novel anode materials for lithium ion batteries

The performance of lithium ion batteries (LIBs) is highly dependent on the properties of the anode materials. Developing new carbon materials and metal oxides/sulfides with high capacities has attracted growing attention due to the limited theoretical capacity of commercial graphite. Pyrite (FeS₂) is environmental benign, inexpensive and has a high theoretical capacity of 894 mAh g^-1 because of its four-electron reduction by lithium, which make it promising for use as an anode material of LIBs. To improve the electrical conductivity and volume change of pyrite, hybrids made of pyrite confined in nitrogen-doped carbon nanotubes (FeS₂/N-CNTs) were fabricated by floating catalyst chemical vapor deposition (FCCVD), followed by sulfidation. Results indicate that the original Fe/N-CNTs formed during the FCCVD have iron nanowires or nanorods inside the N-CNTs and their sulfidation with sulfur vapor at 400℃ for 1, 2 and 5 h leads to FeS₂/N-CNTs with pyrite contents of 22.4, 45.2, and 55.8 wt%, respectively. The pyrite in FeS₂/N-CNTs is in two forms, one is FeS₂ nanowires confined in half-open N-CNTs and the other is FeS₂ nanoparticles attached to the outer walls of the N-CNTs as a result of diffusion out of the inner tubes. A large number of defects on the N-CNTs tube walls is necessary for the diffusion of sulfur vapor into the tubes to make the iron species accessible to the sulfur. The FeS₂/N-CNTs-45.2 wt% has the highest discharge capacity (996 mAh g^-1 at 0.1 A g^-1), good rate capability and stable cycling performance as an anode material for LIBs.