| With widespread usage of portable electronic devices, electric vehicles and large-scale grid energy storage, now there is an urgent demand for the rapid development of high performance and long-life next-generation batteries. Within the class of rechargeable batteries, Lithium-sulfur (Li-S) batteries as one of the most promising candidates have currently attracted worldwide academic and industrial interest because of their high theoretical energy density (2500 Wh kg-1), the high natural abundance, low cost and environmental friendliness of sulfur. However, we still need deeper insight to thoroughly resolve remaining critical challenges for the practical applications of Li-S cells to date, such as incomplete sulfur utilization, fast capacity degradation, and poor Coulombic efficiency of Li-S batteries. In order to modify the cathode material, we prepared three composites through different methods. These three composites are mesoporous MnO2 nanotubes/sulfur, core-shell S@MnO2 nanospheres and Three-dimensional hollow graphene spheres/sulfur (HGs/S).(1) We fabricate the mesoporous hollow MnO2 nantubes as efficient sulfur hosts. To begin with, the self-standing carbon nanofibrous membranes are prepared via electrospinning and high-temperature carbonization, followed by an in situ redox reaction in KMnO4 solution to coat the carbon nanofiber (CNF) template with MnO2 nanosheets until the CNFs are consumed. The synthesized mesoporous hollow MnO2 nanotubes are composed of an inner shell of packed MnO2,and an outer shell of mesoporous sheet like MnO2,which provide physical confinement and chemical interaction and shows excellent efficiency for trapping the polysulfides. The sulfur-mesoporous hollow MnO2 nantubes electrode delivered a initial specific capacity of 967 mAh g-1 at 1 C and maintained a stable cycling performance of 556 mAh g-1 over 300 cycles, a low capacity-decay rate of 0.14 % per cycle.(2) Core-shell structure and interaction mechanism of ?-MnO2 coated sulfur for improved lithium-sulfur batteries. A ?-MnO2-coated solid sulfur nanocomposite was prepared with core-shell structure through the redox reaction between KMnO4 and MnSO4, and then deeply investigated interaction mechanisms of ?-MnO2 and polysulfides in the rechargeable Li-S cells. In comparison with bare solid S nanospheres, the core-shell S@?-MnO2 composites exhibited very good electrochemical performance, with a good capacity retention of 82 % after 300 cycles at a C/2 rate. The capacity decay over 600 cycles at 1C, and over 400 cycles even at 2C are found to be 0.07% and 0.14% per cycle, respectively.(3) We have designed a novel three-dimensional (3D) nanostructure of hollow graphene spheres (HGs) as the sulfur host. The 3D HGs were successfully prepared via a self-assembly method of wrapping graphene oxide (GO) on SiO2 spheres, and then followed by carbonization and etching of the SiO2. The impregnation of sulfur into the hollow graphene spheres lead to obtaining the HGs/S cathode, which reaches up a high sulfur loading of 90 wt%. The HGs/S cathode material remains a high discharge capacity of 810 mAh g-1 after 200 cycles at 0.5 C rate.Furthermore, it demonstrates a low capacity-decay rate of 0.083 % per cycle after 600 cycles at 1 C rate. Compared with pristine reduced graphene oxide/sulfur composites (RGO/S), the as-prepared 3D self-assembled graphene hollow spheres HGs/S exhibit significantly improved electrochemical performances in terms of high specific capacity, remarkable rate capability and excellent cycling stability. These synergistic effects are achieved by more effective 3D ion/electron transport pathways, and efficient confinement of polysulfide dissolution and shuttling. |
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