Multifunctional Co9S8 nanotubes for high-performance lithium-sulfur batteries

The commercialization of Lithium-sulfur (Li-S) batteries is being hampered by the inherent insulation and volume expansion of sulfur, as well as by shuttle effect of polysulfides. To expand the Li-S application, Co9S8@S nanotube composites were developed and fabricated via hydrothermal method combining with incorporating sulfur using melt-diffusion method. In comparison with pure sulfur cathode, the Co9S8@S nanotube composites cathode delivered excellent specific capacity, had remarkable rate performance and superior coulombic efficiency. The initial capacity of Co9S8@S nanotube composites cathode was 937 mAh/g and stabled at 650 mAh/g after 100 cycles at 0.1 C, much superior than pure S electrode. The improved electrochemical performance of Co9S8@S nanotube composites cathode was awarded to highly conductive and polar Co9S8 nanotubes. On the one hand, Co9S8 nanotubes help to form effective conductive networks which can improve the transport of electrons/lithium ion and overall electrical conductivity; on the other hand, polar Co9S8 nanotubes entrapped polysulfides through chemical adsorption to alleviate the shuttle effect. In addition, sulfur particles were well-distributed on the hollow Co9S8 nanotubes, which relieve the volume expansion of sulfur effectively. These synergistic effects were achieved by physical constraints and chemical effects of hollow Co9S8 nanotubes.