Fabrication And Electrochemical Performance Of Functional Carbon Fiber Based Sulfur Host For Lithium Sulfur Batteries

Lithium sulfur batteries have attracted numerous attentions as one of the most promising candidates for the next generation second batteries because of their high specific capacity and low material cost.However,the low conductivity of elemental sulfur and immigration of polysulfide will cause dramatic decrease in electrochemical performance of the batteries.In order to solve these problems,strategies of nitrogen doping,inorganic modification and introduction of functional separator were taken to improve the conductivity and sulfur immobilization ability of the carbon fiber based sulfur host in this work.In addition,the polysulfide trapping mechanism of the sulfur host was also discussed.3D nitrogen doped carbon fiber(NCF)was fabricated with cotton fiber and polyaniline as precursors,in which carbon fiber worked as electron transport network and nitrogen species worked as polysulfide trapping sites.In order to increase the infiltration of electrolyte and transport of Li+ in the cathode,pure sulfur cathode with nano structure was fabricated through sulfur vapor deposition method on the surface of Al foil.When stacked NCF and pure sulfur cathode together,the integrated cathode showed excellent cycling performances.The cathode with sulfur loadlng of 3 mg cm-2 can exhibit a discharge capacity of 534 mAh g-1 after 300 cycles under 1C.After 500 cycles at 2C,the cathode with a high sulfur loading of 3 mg cm-2 delivered 440 mAh g-1 with a high capacity retention of 97%.CoO/Co modified porous carbon fiber(CoO/Co@PCF)was fabricated with cotton fiber and CoS in the condition of calcination under inert atmosphere.CoS can be converted to CoO by oxygen species in the cotton fiber under high temperature,and part of CoO will then react with carbonized fiber to produce metallic Co and pores in the fiber.In the CoO/Co@PCF composite,carbon fiber worked as electrochemical reaction sites for the active materials,the pores in the carbon fiber and inorganic CoO/Co can behave as physical and chemical adsorption sites for the polysulfide respectively.When applied as sulfur host in cathode,the electrochemical performance was dramatically improved.With sulfur loading of 3 mg cm-2,the cathode showed a high discharge capacity of 514 mAh g-1 after 300 cycles under 1C.When the sulfur loading was increased to 5.4 mg cm-2,the cathode can still deliver a discharge capacity of 508 mAh g-1 after 100 cycles under 0.5C with a low capacity fading rate of 0.075%per cycle.In order to improve the sulfur utilization and capacity retention of carbon fiber based cathode,a multifunctional modified separator was fabricated with good film forming ?-MnO2 nanowire and graphene.In the functional separator,?-MnO2 nanowire can anchor polysulfide through chemical bonding and graphene nanosheets can suppress the immigration of polysulfide via physical shielding.Moreover,the conductive graphene can also increase the utilization of sulfur and decrease the polarization of the battery by working as upper current collector for the cathode.The discharge capacity of the carbon fiber based cathodes was nearly doubled under different rates when worked with the multifunctional separator.When the sulfur loading of the cathode was 3 mg cm-2,the cathode showed a high discharge capacity of 530 mAh g-1 after 500 cycles under 1C with a low capacity fading rate of 0.051%per cycle.