Synthesis And Electrochemical Performance Of Functionalized Carbon/sulfur Cathode Materials For Lithium Sulfur Batteries

With the depletion of fossil fuels such as petroleum,the aggravation of environmental pollution,and the rapid development of electric vehicles and energy storage devices,lithium secondary batteries with high energy density are urgent to meet the needs of human and industrial development.The sulfur electrodes for lithium-sulfur battery have been paid extensive attention to due to their high theoretical specific capacity(1672 m Ah?g^-1)and energy density(2500Wh?kg^-1).However,some problems such as poor conductivity of elemental sulfur and lithium sulfide,volume expansion of sulfur electrode during lithiation,and dissolution of polysulfide lead to low utilization of active materials and poor cycle stability,which hinders the industrialization of lithium-sulfur batteries.To address the above problems,this dissertation mainly focuses on synthesizing functionalized carbon/sulfur composites to improve the utilization of active materials and electrochemical performance of sulfur electrode.To restrict the dissolution and diffusion of soluble lithium polysulfides and volume expansion of sulfur electrode,neodymium oxide nanoparticles are doped and polyethylene glycol is grafted on the surface of hybrid carbon.The detail contents are listed as follows.?1?The functionalized aectylene black is used as the conductive carbon skeleton of elemental sulfur.The effect of different contents of acetylene black conductive agent of the electrochemical performance of functionalized acetylene black@sulfur?H-AB@S?electrode is studied.H-AB treated by concentrated nitric acid vapor is used as the carrier of sublimed sulfur.H-AB can limit the dissolution of lithium polysulfide,but H-AB has lower conductivity than the original acetylene black.Therefore,the electrochemical performance of the H-AB@S composite is improved by increasing the content of acetylene black conductive agent.The results illustrate that the H-AB@S composite electrode with 30 wt%acetylene black conductive agent?H-AB@S-30?has a specific discharge capacity of 563 m Ah?g^-1after 100 cycles at a current density of 0.1 C.The capacity retention is 71.2%of the highest discharge specific capacity.?2?A functionalized acetylene black/sulfur composite doped with nano-cerium oxide is synthesized.The electrochemical properties of functionalized acetylene black/sulfur@neodymium oxide?H-AB/S@Nd₂O₃?composites with different doping contents of Nd₂O₃are investigated.Firstly,elemental sulfur is deposited on the H-AB surface,and the H-AB/S@Nd₂O₃composites is synthesized by solution dispersion method.The internal H-ABs framework effectively improves the conductivity of elemental sulfur.The external Nd₂O₃nanoparticles can help fix polysulfides.Thus the synergistic effect of H-AB and Nd₂O₃could suppress the dissolution and diffusion of lithium polysulfides and reduce the shuttle effect during the cycling.The results indicate the H-AB/S@Nd₂O₃composite with 5 wt%Nd₂O₃has a high initial discharge capacity of 1171 m Ah?g^-1at a current density of 0.1 C,and a capacity retention is approximately 63.9%after 200 cycles.Even at a high current densities of 0.2,0.5 and 1 C,the specific discharge capacities remain at 628,492and 475 m Ah?g^-1after 200 cycles,and the capacity retention is 65.4%,59.4%and 68%,respectively.?3?The functionalized acetylene black/carbon nanotubes@sulfur composites is synthesized.And the effects of the content of functionalized carbon nanotubes?H-CNTs?in the mixed carbon and mixed carbon grafted polyethylene glycol?PEG?on the cycle performance and rate capability of the sulfur electrode are investigated.Firstly,the H-AB dispersion and the H-CNTs dispersion are uniformly mixed.Then polyethylene glycol?PEG?is grafted on the surface of the mixed carbon via the esterification reaction.Finally the H-AB/CNTs@S composite has been synthesized via a in-situ chemical deposition.The synergistic effect between functionalized acetylene black and carbon nanotubes with a unique conductive network structure can improve the long-range conductivity of the sulfur electrode.In addition,long-chain PEG can be not only used as an ionic conductor for lithium ion transportation,but also prevent the dissolution of polysulfides during the electrochemical reaction.The results manifest that the H-AB/CNTs@S composite electrode with 30 wt%H-CNTs has an initial discharge specific capacity of 1380 m Ah?g^-1at a current density of 0.1C and remains at 923 m Ah?g^-1after 100 cycles.The capacity retention is 66.9%.Even at a high current density of 0.5 C,the H-AB/CNTs@S composite maintains a specific discharge capacity of 764 m Ah?g^-1after 200 cycles.The capacity retention is 66.6%.?4?Preparation and electrochemical performance of hierarchical porous activated carbon/sulfur composite is studied.The hierarchical porous activated carbon?HPBAC?with large specific surface area and pore volume is obtained under different preparation conditions.Then the HPBAC/S composite has been prepared via melt diffusion method.HPBAC with large specific surface area and pore volume could increase the loading of sulfur.The HPBAC with interconnected hierarchical structure contributes to the rapid transport of Li⁺and electrons.In addition,HPBAC containing functional groups with microporous and mesoporous structure can limit the reaction of sulfur and polysulfide to the inside of the electrode,and improve the utilization of active materials during the charge-discharge process.At a current density of 100 m A?g^-1,the highest specific discharge capacity of the HPBAC/S composite with a sulfur content of74.9 wt%is 1239 m Ah?g^-1.After 200 cycles,the specific capacity of the HPBAC/S composite is 722 m Ah?g^-1,and the capacity retention is 58.3%.