Doping Modification Of Carbon Materials And Their Applications In Lithium-Sulfur Battery

In recent years,lithium-sulfur battery is considered to be one of the most promising next generation energy storage systems due to its ultra-high theoretical specific capacity(1675 m Ah g^-1)and theoretical energy density(2600 Wh kg^-1).However,lithium-sulfur battery has not been applied in large-scale commercial field.Because there are still a lot of issues in lithium-sulfur battery have not been dissolved,such as the insulation of elemental sulfur and its discharge products,the volume change of the cathode and the“shuttle effect”of polysulfides during charge and discharge processes.In order to solve the problems of lithium-sulfur battery mentioned above,this thesis did researches to improve the electrochemical performance of lithium-sulfur battery through structure design on the cathode and modification of separator.The research content is as follows:1.Using sodium lignosulfonate and melamine as precursors,sodium chloride as template,a nitrogen-doped porous carbon（NPC）is prepared by freeze-drying and high-temperature carbonization processes.The morphology and structure of NPC can be designed and controlled by adjusting the state of the precursor and the temperature during the carbonization process.The optimized NPC has a complex porous structure,which has improved the conductivity of the cathode and alleviate volume expansion.The doping of N element further enhances the inhibition of“shuttle effect”.Benefiting from the above advantages,the battery assembled with this cathode delivers a specific discharge capacity of 677.4 m Ah g^-1 after 200 cycles at a current density of 0.5 C.2.Cobalt-doped carbon nanotubes modified reduced graphene oxide aerogels（rGO+CNT@Co）were prepared by high-temperature carbonization,freeze-drying and self-propagating at high temperature.rGO+CNT@Co is a three-dimensional conductive network formed by cross-linking of lamellar rGO and tubular CNT@Co,which can not only accommodate more sulfur and improve the conductivity of cathode,but can also adsorb polysulfides physically and chemically.rGO+CNT@Co can be used as conducting matrix for freestanding cathode and normal cathode,respectively.When it is used in freestanding cathode,it has the advantages of flexibility,high sulfur loading and delivers excellent cycle stabilities.When it is used in normal cathode,it performs good rate capabilities.The battery obtains a reversible discharge capacity of 517.9 m Ah g^-1 even at high rate of 2 C.3.Using melamine as the carbon source and cobalt nitrate hexahydrate as the cobalt source and catalyst,cobalt-doped carbon nanotubes（CNT@Co）are synthesized by high-temperature carbonization,then CNT@Co is loaded on PP separator by vacuum filtration.The CNT@Co layer acts as second current collector,which can improve the utilization of active materials.In addition,the Co element can chemically adsorbed polysulfides and catalyzes polysulfides.Therefore,the battery assembled with CNT@Co modified PP separator delivers good electrochemical performance and can maintain a reversible specific capacity of 530 m Ah g^-1 after 100 cycles at a current density of 0.5 C.