| Lithium-sulfur (Li-S) batteries have been recognized as one of promising high performance rechargeable lithium batteries because of the high theoretical specific capacity (1675 mAh·g-1), low cost, environmental friendliness, and abundant resources of sulfur. However, their practical use is still hindered due to the electronically and ionically insulating nature of the pristine sulfur and discharge product Li2S resulting in low utilization and poor rate capacity of sulfur cathode, and the dissolution of soluble lithium polysulfides intermediates generated during cycling leading to fast capacity fading and high self-discharge, and the large volume change of sulfur during cycling giving rise to serious pulverization of the electrode. Herein, aiming at the problems mentioned above, graphene oxide (GO), three-dimensional grapheme (3DG) and nitrogen-doped grapheme (NG) were applied in cathode materials of Li-S batteries in this work.(1) Graphene oxide/sulfur (S@GO) composites were prepared by the self-assembly of the cationic surfactant stabilized sulfur particles and negatively charged graphene oxide nanosheets through electrostatic attraction. Due to the small size(1-2 ?m) of spherical sulfur particles that were well wrapped by the GO nanosheets, the S@GO-1 composite showed a high initial discharge specific capacity (1061.6 mAh·g-1) at a current rate of 0.1 C, and good cycling stability with 56.7% capacity maintenance over 50 cycles. Furthermore, a high coloumbic efficiency of ca.98% was also obtained during cycling.(2)Three-dimensional graphene/sulfur (3DG-S) composites were synthesized by a facile one-pot hydrothermal strategy which integrated the hydrothermal reduction of GO and its self-assembly with sulfur nanoparticles (ca.10-40 nm) that provide short lithium ion diffusion distances. And the composites display a 3D porous conducting network structure constructed by reduced graphene oxide, which improves the kinetics of charge and ion transfer in the 3DG-S electrodes. Moreover, the residual oxygen-containing functional groups on reduced graphene oxide exhibit the good absorption ability with sulfur/polysulfides, which effectively lessens the dissolution of polysulfide intermediates into the electrolyte and improve the cycling stability. Therefore, the 3DG-S composites demonstrate excellent electrochemical performance. After 100 cycles, the 3DG-S-1 composites showed a high reversible capacity of 583.1 mAh·g-1 and high capacity retention of 72.3%at a current rate of 0.5 C. In addition, the 3DG-S-1 composites exhibited a high coulombic efficiency (>98%) and good rate performance (ca.680 mAh·g-1 at 1 C and ca.620 mAh·g-1 at 2 C).(3) To further improve electrochemical performance of sulfur cathode materials, we prepared nitrogen-doped graphene/sulfur (NG-S) composites. Three types of nitrogen sources were used to synthesize NG through a simple one-pot hydrothermal process. Nitrogen doping and reduction of GO were completed synchronously. Subsequently, chemical deposition or thermal compounding methods were used to incorporate sulfur with NG to fabricate several kinds of NG/S composites. The NG-1W/S composite with high loading (75.5 wt%) of sulfur could achieve a high discharge specific capacity of 723.9 mAh·g-1 and exceptional cycling stability with high capacity retention of 87.4% and coulombic efficiency of ca.98% at a current rate of 0.7 C after 100 cycles, as well as high specific discharge capacities at higher rates (ca.780 mAh·g-1 at 1 C and ca.610 mAh·g-1 at 3 C). The outstanding electrochemical properties could be attributed to the good incorporation of NG-1W and sulfur particles, improved electronic conductivity of graphene framework by nitrogen doping, and strong lithium polysulfides adsorption capability of the doped nitrogen atoms of NG-1W. The graphene frameworks also can alleviate the volume effect during in charge/discharge process. Additionally, the high ratio of the pyridinic-N to overall nitrogen makes the NG-1W/S composite have better lithium polysulfides adsorption capability. |
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