Preparation Of Cathode And Modified Separator Using High Conductivity Carbon Material And Performance Research In Li-S Battery

Lithium sulfur（Li-S）batteries have been regarded as the next generation of advanced battery system with high energy density due to its ultrahigh theoretical specific capacity of 1675 m Ah g^-1 and energy density of 2600 Wh kg^-1,low-cost and environmentally friendly.However,the commercialized application of the Li-S batteries is still limited by low electrical conductivity of active sulfur,the notorious shuttle effect of lithium polysulfides during the discharge/charge as well as large volume expansion of sulfur cathode.Highly conductive porous carbon materials not only effectively improve the electronic conductivity of sulfur cathode and alleviate the volume expansion,but also restrict the shuttle effect by physical absorption.Moreover,using transition metallic oxide to modify commercial separator as a barrier restrain polysulfide is also considered as an effective way to mitigate the shuttle effect.Herein,natural silk cocoon derived high graphitized N-doping porous carbon is prepared,and the bifunctional separator is constructed by coating conductive Super P and polar nano-Ti O₂ on the commercial separator,the synergistic adsorption and transformation of polysulfide were investigated.In addition,this work prepares self-supporting porous carbon materials using makeup cotton as a carbon source and modified with graphene and carbon nanotubes to construct the cathode host material of high preformance Li-S batteries.The main research contents and results are as follows:（1）High graphitized N-doping porous carbon was prepared via an activation/carbonization approach using silk cocoon as carbon source and Fe Cl₃/Zn Cl₂ as activating agent.The carbon with High degree of graphitization can effectively improve the electron conductivity of cathode and promote the rapid conversion of polysulfide.And the modified separator is prepared using Super P and nano-Ti O₂ by traditional coating method.The effect of Ti O₂ content in modified separator on the coating structure and electrochemical performance of Li-S batteries was investigated.The results show that the adsorption of polysulfide by the modified separator is weak when the amount of Ti O₂ is few.Nevertheless,when the amount of Ti O₂ is large,the polysulfide can be effectively adsorbed,but the poor conductivity of Ti O₂ will blocking the transfer of electrons.Through comprehensive analysis of the morphology and electrochemical performance,the most suitable Ti O₂content is 5 wt%.The corresponding electrode released the initial discharge capacity of 1311 m Ah g^-1 at 0.2 C and maintained the reversible capacity of 901 m Ah g^-1after 350 cycles.（2）The self-supporting porous carbon material was prepared by activation/carbonization method using makeup cotton as carbon source and KHCO₃as activating agent.And the self-supporting porous carbon material with graphene modification improve electronic conductivity of cathode.The effect of graphene content on cathode material and electrochemical performance of was investigated.The results show that the electrochemical performance of Li-S battery is better with the increasing amount of graphene.In addition,the unique lamellar structure of graphene provides more active sites for capturing polysulfide and improves the mechanical properties of the self-supporting electrode.The corresponding electrode show the initial discharge capacity of 949 m Ah g^-1 at 0.2 C and maintained the reversible capacity of 904 m Ah g^-1 after 100 cycles.（3）Makeup cotton derived self-supporting porous carbon material was modified by carbon nanotube（CNTs）to prepare cathode of Li-S batteries.The effect of CNTs suspension concentration on the cathode structure and electrochemical properties was investigated.The results show that when the CNTs suspension concentration is relatively small,the electronic conductivity of the cathode material cannot be effectively improved,but the high CNTs suspension concentration can effectively improve the electronic conductivity of the cathode material,but it will significantly increase the mass of cathode material and reduce the quality energy density.Meanwhile,CNTs can improve the mechanical properties of cathode materials and provide sufficient active sites for storage of active sulfur.Through comprehensive analysis of the morphology and electrochemical performance of the as-prepared cathode,the most suitable CNTs solution concentration is 0.050 mg m L^-1.The corresponding cathode with the sulfur loading of 2.5 mg cm^-2 shows the in initial discharge capacity of 897 m Ah g^-1 at 0.5 C,and retains the reversible capacity of 593 m Ah g^-1 after 300 cycles.Even the sulfur loading increase to 8.0 mg cm^-2,the cathode still releases the discharge capacity of 455 m Ah g^-1after 100 cycles.