Simultaneous growth of carbon nanotubes on inner/outer surfaces of porous polyhedra: Advanced sulfur hosts for lithium-sulfur batteries

Metal-organic framework (MOF)-derived functional carbon matrices have recently attracted considerable attention as energy-storage materials. However, the development of MOF-derived carbon materials with hierarchical structures, capable of thoroughly preventing the "shuttling" of polysulfides, is still a major challenge. Herein, we synthesized cobalt nanoparticle-containing porous carbon polyhedra with in situ grown N-doped carbon nanotube (CNT) backbone (NCCNT-Co), using zeolitic imidazolate framework-67 (ZIF-67) as starting material. The obtained NCCNT-Co, with interconnected N-doped CNTs on both inner and outer surfaces, possesses an integrated conductive network, which can further accelerate the transport of electrons/ions inside the whole sulfur cathode. The mesoporous structure derived from the ZIF-67 matrix and the densely immobilized CNTs, coupled with the homogeneously doped N atoms and Co nanoparticles, can efficiently trap lithium polysulfides (LiPSs) by physical confinement and chemical interactions. Furthermore, the hierarchical structure of the porous carbon polyhedra enables a high sulfur loading of up to 76 wt.% and can also buffer the volume changes of active sulfur during the lithiation process. As a result, the NCCNT-Co-S cathode delivers a high initial specific capacity of 1, 300 mAh·g^-1 at 0.1 C, along with a high capacity of 860 mAh·g^-1 after 500 cycles at 1 C, with an extremely low capacity decay of 0.024% per cycle.