摘要/Abstract

锂硫电池具有理论能量密度高、活性物质价廉、毒性低等优点，是最具发展潜力的高能量二次电池之一，其应用仍存在硫面载量小、循环寿命短和库伦效率低等难题.制备了石墨烯包覆的硫填充碳纳米笼自支撑整体材料，可直接用作锂硫电池正极，避免使用粘结剂、导电剂和集流体，当硫的面载量为3.8 mg·cm^-2时，锂硫电池展现出高的可逆比容量（1104 mAh·g^-1）、优异的循环稳定性（每圈容量衰减率仅为0.049%@1.0 A·g^-1）和>99.9%的库伦效率，其面积比容量（3.7 mAh·cm^-2）处于锂硫电池的先进水平.该电极的优异性能可归因于以下因素的协同作用：碳纳米笼的物理限域作用及石墨烯中含氧官能团的化学吸附作用有效抑制了活性物质的流失，微孔-介孔-大孔共存的分级孔结构和高导电性利于离子和电子的传输，纳米笼空腔填充有利于缓解体积膨胀造成的影响，整体材料的自支撑稳定结构有利于增加硫载量且维持电化学性能.本研究还提供了一种工艺简单、能有效提高面积比容量的硫正极制备方法.
关键词: 锂硫电池, 整体材料, 物理限域, 化学吸附, 循环稳定性, 面积比容量, 高硫载量
Lithium-sulfur (Li-S) batteries have attracted considerable attention due to their high theoretical energy density, low cost and low toxicity of active materials. Despite the great progress achieved recently, the practical application of Li-S batteries still faces several critical challenges, i.e, low capacities, low coulombic efficiency and rapid capacity decay during cycling. These problems mainly originate from the reasons of:(i) the poor intrinsic electrical conductivity of sulfur and Li₂S decreasing the utilization of sulfur, (ii) the shuttle effect of soluble polysulfide intermediates losing active components, (iii) the large volumetric change pulverizing electrode during cycling, which become more and more serious when the sulfur loading is increased to the practical level of 3~5 mg·cm^-2. To address these issues, some approaches have been developed, including hybridizing sulfur with highly conductive materials to enhance the conductivity, encapsulating sulfur in porous materials to inhibit the loss of polysulfides and accommodate the volumetric expansion, blending sulfur with polar materials to restrain the diffusion of polysulfides by chemical interaction. Recently, our group reported the mesostructured cathode material of sulfur-filled carbon nanocages (S@hCNC), which demonstrated the large capacity, high-rate capability and long cycle life owing to unique mesostructured feature, physical confinement, good conductivity and coexisting micro-meso-macropores. However, the non-polar sp² carbons only have weak interaction with polar polysulfides. The introduction of chemical adsorption sites for polysulfides through heteroatom doping or surface modification can obviously enhance the interaction between the host and lithium polysulfides, thus further improving the cycling stability. Herein, we report the free-standing monolithic materials of reduced graphene oxide wrapped S@hCNC (S@hCNC@rGO), which can be directly used as the cathode without using binders, conductive materials and current collector. The S@hCNC@rGO battery with the high areal sulfur loading of 3.8 mg·cm^-2 delivers excellent electrochemical performances surpassing the counterpart of S@hCNC, e.g., high reversible specific capacity (1104 mAh·g^-1@0.2 A·g^-1), superior cycle stability (low degradation rate of 0.049% per-cycle@1.0 A·g^-1), high coulombic efficiency (>99.9%) and the top-ranking areal capacity of 3.7 mAh·cm^-2. Such excellent electrochemical performances can be ascribed to the synergism of the physical confinement of hCNC, the chemical adsorption of oxygen functional groups on rGO, the accelerated charge transfer kinetics arising from the hierarchical porous structure and high electrical conductivity, and the free-standing structure with high stability. This study also suggests a simple and efficient method to develop sulfur cathode with high areal capacity, which pave a way for the practical application of Li-S batteries.
Key words: lithium-sulfur batteries, monolithic materials, physical confinement, chemical adsorption, cycling stability, areal capacity, high sulfur loading


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