Modification,Structural Design And Electrochemical Performance Of Sulfur Cathode For Lithium-sulfur Batteries

Lithium sulfur batteries have the advantages of high specific energy?theoretical energy density of 2600 Whkg–1and 2800 Wh L–1?,high capacity,environmental friendly and low cost.However,despite these promising advantages mentioned before,sulfur cathodes still suffer from several inherent challenges that hinder their commercialization.For example,Li–S batteries are known to suffer from low utilization of active material and rapid capacity decay during the cycling process.The main reason causing the above problem is low electronic/ion conductivity of sulfur and its discharge products.In addition,the notable ?shuttle effects? caused by the dissolution of polysulfides in the organic electrolyte,combined with the large volumetric expansion during cycling as well as the uncontrolled and irreversible deposition of Li2S2/Li2 S are believed to be the main reasons for the low coulombic efficiency and rapid capacity fade for Li–S batteries.This dissertation extensively investigates on the nano-structure design of sulfur electrode and electrode interface modification as well asimproving the uncontrolled and irreversible deposition of Li2S2/Li2 S,thus alleviating the short cycling life and low specific capacity of lithium sulfur battery.1.Interconnected core–shell nanocomposites of pyrolyzed polyacrylonitrile@carbon/sulfur?pPAN@C/S?have been prepared as by cathode materials for high-performance Li–S batteries by coating and annealing of polyacrylonitrile?PAN?on C/S composites.For the pPAN@C/S composites,the conductive porous carbon could improve the electrical conductivity.Meanwhile,the porous carbon and the pPAN polymer shell could not only act as small nanoreactors to restrict the dissolution of lithium polysulfides in organic electrolytes,but also alleviate volume expansion during the cycling process.Moreover,the pPAN polymer shell further guarantees a more reversible charge/discharge process of the active material by chemical binding to polysulfides.Benefitting from both the entrapment of the polysulfides by the carbon core and the pPAN polymer shell,this interconnected core–shell pPAN1@C/S electrode exhibited a very high initial capacity of 1269 mAh g–1 at 0.5 C.Excellent cycling stability and rate performance were also obtained by employing pPAN1@C/S composites as electrode materials.2.A special N-doped ZIF-8-derived carbon nanosphere?N-ZDC?with hierarchically porous inner architecture was prepared as sulfur host material for high-performance and long-term lithium–sulfur batteries.The hollow space of porous N-ZDC ensures a high sulfur loading and accommodates volume expansion of sulfur species during cycling process.The special complex inner structure of N-ZDC serves as multilayered physical barrier against polysulfides dissolution into the electrolyte,also increases the electrical contact of sulfur species with carbon host and consequently highly improve sulfur utilization.Moreover,nitrogen doping provides chemical binding between the host and sulfur species to further effectively trap the polysulfides.As a result,the N-ZDC4/S electrodes show high initial specific capacities of up to 1343 mAh g^-1 at 0.5 C,1182 mAh g^-1 at 1 C,and 698 mAh g^-1 at 2 C,and ultraslow capacity decay of only 0.063% per cycle at 2 C over 800 cycles,indicating high rate capability and excellent long cycling stability.3.Electrocatalysis principleswas introduced to Li-S battery in order to improve the electrochemical performance of sulfur cathodes.By rational designing the multifunctional nitrogen doped carbon based sulfur host material Co-HC-NCNTs,we preliminarily investigated the influence of nitrogen doping and uniform presence of cobalt nanoparticles for the reversible deposition of Li2S2/Li2 S.As a result,the utilization of sulfur species was largely improved,and the advantages of the electrode were fuether studied.the Co-HC-CNTs/S electrodes show high initial specific capacities up to 1225 mAh g^-1 and maitain at about 1112 mAh g^-1 at 0.5 C after 300 cycles.Even at a higher current rate of 1 C,the initial specific capacities up to 902 mAh g^-1 and maitain at about 776 mAh g^-1 after 300 cycles.And a ultraslow capacity decay of only 0.046% per cycle indicating high rate capability and excellent long cycling stability.4.The CC@Co₃O₄ nanoneedle array with a bottlebrush-like structure was employed as an efficient multifunctional ?super-reservoir? to prolong the cycle life of Li–S batteries.By using ex-situ Raman and other electrochemical characterization methods,the role of Co₃O₄ nanoneedle array in the a deposition/decomposition of Li2S2/Li2 S had been extensively investigated.A high initial capacity of 1231 mAh g^-1 at 0.5 C and a slow capacity decay of 0.049% per cycle at 2.0 C for 500 cycles were achieved.Excellent rate performance was also obtained.This work demonstrates that the concept of employing catalytic host material will improve the electrochemical performance for high-energy Li–S batteries.