Construction And Electrochemical Performance Of Nanomaterials For Lithium Sulfur Battery Cathode

In recent years,the demand for renewable energy is growing.Clean energy has been paid more attention due to the rapid development of modern industry and agriculture,which makes the high-efficiency energy storage technology to be unprecedentedly developed.Among various energy storage systems,lithium sulfur battery(Li-S)has attracted much attention due to its high energy density(2600 Wh kg^-1)and high theoretical specific capacity(1675 m Ah g^-1)of sulfur.Due to the insulation of sulfur and lithium sulfide,the shuttle effect of intermediate lithium polysulfide,and the volume expansion of sulfur during charging and discharging,the practical application of lithium sulfur battery is seriously restricted.Reasonable design and construction of cathode materials with special structure and morphology can solve the problems of lithium sulfur battery and improve its electrochemical performance.In this dissertation,Co/N-C composites,FeCo/N-C composites and Ni/NiO composites are designed and constructed,and their structures,morphologies and electrochemical properties are detailedly studied.The details are as follows:1.Co/N co-doped carbon nanotube composites are prepared by a simple one-step pyrolysis method.The existence of Co nanoparticles can catalyze the conversion of polysulfides,thus inhibiting the shuttle effect and improving the reaction kinetics of lithium sulfur battery.Hollow carbon nanotubes can not only improve the conductivity,but also buffer the volume expansion of sulfur during the cycle.Pyridine nitrogen and pyrrole nitrogen are used as excellent anchoring sites to adsorb soluble polysulfides and improve the kinetics of electrochemical reaction.When the prepared materials are used as cathode materials for lithium sulfur battery,the initial discharge capacity is 1016 m Ah g^-1 at 0.1 C current density,the reversible specific capacity is 411 m Ah g^-1 after 450 cycles,and the capacity decay rate of each cycle is 0.13%.2.FeCo/N co-doped carbon nanofiber composites are fabricated by electrospinning and subsequent calcination process,which are used as cathode materials for lithium sulfur battery to improve the electrochemical performance.Because of its high specific surface area,porous carbon nanofibers can not only increase the contact area between active sulfur and the host material,but also improve the conductivity of the electrode.FeCo alloy is mainly used to catalyze the conversion of polysulfides,and the metal alloy not only has the advantages of a single metal,but also can further improve the electrochemical performance of lithium sulfur battery due to the synergistic effect between the two metals.At 0.1 C current density,the initial discharge capacity is 1067 m Ah g^-1,and the specific discharge capacity is 1012 m Ah g^-1 during the second cycle.After 300 cycles,the capacity decay rate of each cycle is 2.3%.3.Ni/NiO nanowire-in-nanotube structured nanofibers composites are prepared by electro-spinning and followed by two-stage oxidation treatment.Ni is used to catalyze the conversion of polysulfides,and NiO can adsorb the intermediate polysulfides by chemical action.When the material loaded active sulfur is used as the cathode of lithium sulfur battery,the first discharge capacity of the cathode material is 1073 m Ah g^-1 at 0.1 C current density.After 200 cycles,the reversible specific capacity maintains at 441 m Ah g^-1,the capacity decay rate is 2.9%,and the coulomb efficiency remains above 98%.