Hui Zhang1

1, Peking University, Beijing, , China

Lithium sulfur battery represents an advanced energy storage system because of its environmental benignity, high theoretical energy density (2600 Wh kg-1) and natural abundance of sulfur. However, the low conductivity of sulfur, polysulfides dissolution, and sulfur volumetric expansion during lithiation/delithiation process will decrease the reaction kinetics of polysulfides and sulfur utilization, leading to low capacity, limited rate capability and inferior cycling stability. To solve these problems, there have been intensive efforts in choosing highly conductive carbon materials to design porous or hollow structures for secure sulfur hosts. Although these materials can effectively alleviate polysulfides dissolution during short-time cycling, the weak interaction with polar polysulfides will inevitably results in increase of charge transfer and dissolution of polysulfides over long-time electrochemical reaction.

Here, we first grew ZIF-67 nanoparticles (~450 nm) uniformly within a three-dimensional carbon nanotube sponge, and after carbonization and sulfuration, we finally fabricated a hybrid network with numerous carbon nanotubes penetrating hierarchically porous graphitic carbon polyhedrons uniformly dispersing Co3S4 nanoparticles (from ZIF-67) to host sulfur. Co3S4 has been reported to have strong affinity to polysulfides, and can act as a catalyst to accelerate the conversion of polysulfides to Li2S2/Li2S (insoluble discharged products), which are beneficial for improving battery cycling stability (polysulfide stabilization) and rate capability (increased reaction kinetics). In our system, highly dispersed Co3S4 nanoparticles can provide sufficient sites to trap polysulfides and catalyze the conversion reaction smoothly, additionally the outer wrapping porous graphitic carbons (physical barriers) can further protect polysulfides from dissolving into the electrolyte. Moreover, the highly three-dimensional conductive CNT and graphitic carbon hybrid network acting as a self-standing electrode can not only facilitate electrolyte infiltration and charge transport, but also improve sulfur loading and utilization. Our novel 3D hybrid electrodes exhibit a much superior ultralong Li-S battery performance than previously reported MOF-based electrodes in recent literature.