Carbon Nanotube-Connected Yolk-Shell Carbon Nanopolyhedras with Cobalt and Nitrogen Doping as Sulfur Immobilizers for High-Performance Lithium-Sulfur Batteries

The lithium sulfur battery is regarded as a promising energy solution because of its high energy density. However, the insulating nature and large volumetric expansion of sulfur and the high solubility of polysulfides restrict their practical applications. Here carbon nanotube (CNT)-induced yolk-shell carbon nanopolyhedra, with Co-N-doping, is used as host material for sulfur. The CNTs are used to create a conductive network which interweaves each carbon polyhedron and induces the formation of a yolk-shell structure during the sulfur melt-diffusion process due to the "perforation effect". The CNT-connected Co-N-doped carbon nanopolyhedra containing sulfur yolk-shell structure (S@Co-N-C/CNTs-0.5) can achieve a capacity of 712.2 mAh g^-1 at 1675 (1 C) mA g^-1 after 300 cycles and 511.8 mAh g^-1 at 3350 (2 C) mA g^-1. The outstanding performance is attributed to the new paradigm, S@Co-N-C/CNTs-0.5 yolk-shell structure, which creates a conductive network allowing for improved electron transport and convenient electrolyte infiltration, as well as enhanced reaction kinetics for the electrochemical process synchronously. The significant internal void space of yolk-shell structure effectively accommodates the volume expansion of sulfur. Simultaneously, Co-N-doping in yolk-shell structure carbon polyhedra can synergistically trap polysulfides due to the strong chemical adsorption.