Construction And Performance Of Li-S Battery Cathodes With Synergism Of Physical Confinement And Chemical Adsorption

Lithium-sulfur(Li-S)batteries have become one of the most promising secondary battery systems due to their high theoretical energy density,low cost and low toxicity of active materials.Despite the great progress achieved recently,the large-scale applications of Li-S batteries still confront several challenges such as low areal specific capacity and poor cycling performance.These shortcomings are mainly due to the poor intrinsic electrical conductivity of sulfur and Li2S,the shuttle effect of soluble polysulfide intermediates,and the large volumetric change pulverizing electrode during cycling.To address these issues,carbon-based materials,conductive polymer and metal compounds have been developed to hybridize sulfur.Carbon-based materials have attracted wide attention for their light weight,good conductivity,rich pore structure,large specific surface area,easy doping or chemical surface modification.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 cycling 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,which could further improve the cycling stability.In this thesis,I concentrate on the design,controllable preparation and electrochemical performances of the cathode materials of Li-S batteries with synergism of physical confinement and chemical adsorption,and has achieved the following progress:1,The preparation and electrochemical performances of self-supporting monolithic materials of reduced graphene oxide wrapped sulfur-filled carbon nanocages for Li-S batteries:On the basis of the S@hCNC,we prepare the self-supporting monolithic materials of reduced graphene oxide wrapped S@hCNC(S@hCNC@rGO).S@hCNC@rGO integrates the functions of physical confinement and chemical adsorption,which can effectively inhibit the loss of polysulfide intermediates and facilitate the synergic transport of electrons and ions.With a high areal sulfur loading of 3.8 mg cm-2,the Li-S@hCNC@rGO battery exhibits excellent electrochemical performances surpassing the counterpart of Li-S@hCNC,e.g.,superior cycling stability(low degradation rate of 0.049%per-cycle@1.0 Ag-1),high coulombic efficiency(>99.9%)and the top-ranking areal capacity of 3.7 mAh cm-2.Such excellent electrochemical performances can be attributed 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 self-supporting structure with high stability.In addition,S@hCNC@rGO can be used directly as the electrode without the additives and the current collector,which is very convenient for battery assembly.2,The preparation and electrochemical performances of the sulfur-filled hierarchical nitrogen and sulfur co-doped carbon nanocages for Li-S batteries:The hierarchical nitrogen and sulfur co-doped carbon nanocages(hNSCNC)was prepared by in situ MgO template method with pyridine/thiophene precursor.The Li-S battery based on the sulfur-filled hNSCNC exhibits better electrochemical performances than those based on nitrogen-doped,sulfur-doped and non-doped hCNC,e.g.,the high initial discharge specific capacity(1382 mAh g-1@0.2 Ag-1),superior cycling stability(low degradation rate of 0.09%@1.0 A g-1 and 0.10%@2.0 A g-1),high coulombic effciency(close to 100%).These excellent performances can be ascribed to the synergism of:1)the physical confinement of the nanocages and the multi-site synergistic chemical adsorption of nitrogen and sulfur heteroatoms for inhabiting the shuttle effect of polysulfide intermediates;2)the high electrical conductivity and hierarchical porous structure for facilitating the synergic transport of electrons and ions;3)the confinement of the nanocages for alleviating the pulverization of the electrode structure during cycling.These results reveal the significant synergistic effect of physical confinement and chemical adsorption on the specific capacity and cycling performance of Li-S batteries,which could provide useful reference for the design and development of high-stable Li-S batteries.