Structural Design,Interface Engineering And Electrochemical Performance Of Sulfur Cathode For Lithium–Sulfur Batteries

Lithium–sulfur(Li–S)batteries,using elemental sulfur as the cathode and metal lithium as the anode,exhibit a high theoretical energy density of 2600 Wh kg–1 and 2800 Wh L–1.In addition,elemental sulfur also shows advantages of natural abundance,low cost and environmental friendliness.Therefore,Li–S batteries are currently one of the most potential secondary batteries.Meanwhile,the rapid development of electric vehicles,smart grid and near space vehicles demands significant improvement of rechargeable battery with higher energy densities.In this regard,Li–S battery has emerged as a prospective candidate owing to their higher theoretical energy density.Despite these promising advantages,many challenges still remain in the widespread practical realization of Li–S batteries,such as low energy density and poor cycling performance.Aiming to improve the specific capacity and cycling stability of sulfur electrode,this dissertation investigates extensively on the design of sulfur electrode architectures and engineering the electrode interface,thus alleviating the problems of low electronic/ionic conductivity,easy dissolution of intermediate polysulfides and severe volumetric expansion of sulfur electrode.1.Hierarchically ordered porous carbon was prepared via in-situ self-assembly technology and was used to construct a hierarchically ordered porous carbon /sulfur electrode,with mesopores for confining sulfur and macropores for electrolyte diffusion.The results indicate that the hierarchically ordered porous carbon dramatically enhance the electronic/ionic conductivity of sulfur electrode,accommodate the volume expansion,and suppress the diffusion and shuttle of polysulfide species during charge/discharge process.Electrochemical tests reveal that the porous carbon/S electrode delivers a high initial specific capacity up to 1193 mAh g–1 and a stable capacity of 884 mAh g–1 after 50 cycles at 0.1 C.2.We reported an efficient strategy to confine active sulfur in the porous graphene,which was prapared via a modified chemical activation route.The specific surface area and pore volume of as-prepared porous graphene reach 2313 m2 g-1 and 1.8 cm3 g-1,respectively and the sulfur content in the porous graphene/sulfur composite is up to 67 wt%.The dense nanopores of porous graphene minimize polysulfides dissolution and shuttling in the electrolyte,and thus decrease the loss the active material and suppress the irreversible deposition of discharged products Li2S2/Li2 S on the surface of electrode.The electrochemical tests display the porous graphene/sulfur electrode exhibits a high specific capacity and excellent cycling stability.At a current rate of 1 C,the porous graphene/sulfur electrode delivers a high initial specific capacity up to 927 mAh g–1 and shows high capacity retention of 74%after 100 cycles.3.A self-sacrificial template-directed synthesis method by using zinc oxide nanosheets solid as the self-sacrificial template and Zn2+ resource was proposed to preapred two-dimensional ZIF-8 and ZIF-8-derived carbon nanosheets.With this synthesis route,the thickness of the microporous carbon wall and size of the mesopore could be controlled by adjusting the reaction time.Due to the high specific surface area and hierarchical porosity,when used as host material for confining elemental sulfur,the carbon nanosheets could dramatically minimized the dissolution and shuttle of polysulfides.Moreover,the chemical interaction between the nitrogen atom in the ZIF-8-derived carbon nanosheets and polysulfides further suppress the diffusion of polysulfide,thus improving the specific capacity and cycling stability of sulfur electrode.At a current rate of 0.5 C,the irreversible capacity is still up to 663 mAh g–1 after 100 cycles.4.A sandwich nanostructured Graphene/TiO2 composite and TiO2 nanocrystals were used as the host materials for confining elemental sulfur.Electrochemical tests indicate that both of Graphene/TiO2 composite and TiO2 nanocrystals dramatically enhanced the packing density and cycling stability of sulfur electrode.The results revealed that the pore absorption and chemical interaction between the TiO2 nanocrystals and polysulfides suppress the diffusion and shuttle of polysulfide species in the organic electrolyte.Meanwhile,the in situ formed LixTiO2,which could work as a mixed ionic/electric conductor,synergistically works with highly conductive graphene layer to accelerate Li+/e-transport of sulfur and facilitate the electrochemical reaction of sulfur electrode.At a current rate of 0.5 C,Graphene/TiO2/S and TiO2/S electrodes deliver high initial specific capacities of 985 and 684 mAh g-1,respectively.Even after 100 cycles,the capacity retentions of Graphene/TiO2/S and TiO2/S electrodes are up to 79% and 89%,respectively.5.A vapor-liquid interfacial copolymerization strategy was proposed to prepared Graphene/polymeric sulfur composite.Electrochemical tests suggest that Graphene/polymeric electrode exhibits dramatically improved electrochemical performances as compared with Graphene/sulfur electrode.At a current rate of 0.5 C,Graphene/polymeric sulfur electrode still deliver an irreversible capacity of 635 mAh g–1 after 100 cycles.The uniform dispersion of polymeric sulfur on the surface of graphene permits rapid electronic/ionic transport and suppress polysulfide species diffusion during the charge/discharge process.Furthermore,the organic sulfur units dispersed in the insoluble/insulating Li2S2/Li2 S phase could prevent its irreversible deposition.Based on this,encapsulating polymeric sulfur into porous carbon matrix was also presented.At a current rate of 0.5 C,the porous carbon/polymeric sulfur electrode delivers a high initial specific capacity up to 1105 mAh g–1 and a high capacity retention of 80.5%after 100 cycles.In addition,the porous carbon/polymeric sulfur electrode also exhibits excellent rate performance.At a high current rate of 5 C,it still shows a high reversible capacity of 681 mAh g–1.