Preparation And Electrochemical Properties Of Cathode Nanostructured Materials For Lithium-sulfur Batteries

Lithium-sulfur batteries have attracted extensive attention because of their high theoretical energy density,cost-effectiveness,green environmental friendliness and other advantages.However,there are many difficult problems in lithium-sulfur batteries,such as the insulation of sulfur and Li₂S₂/Li₂S formed by reaction,the shuttle effect of lithium polysulfide and the volume change of sulfur during charge/discharge process,which will seriously lead to the degradation of the electrochemical performance of lithium-sulfur batteries.It has become one of the important research directions in the field of lithium-sulfur batteries via rational design and construction of new cathode materials to obtain lithium-sulfur batteries with excellent rate performance and long cycle life.The specific contents of this thesis are as follows:1.PAN/PANI/chitosan aerogel（abbreviated as PPCA）composites were prepared by chitosan and electrospun PAN/PANI nanofibers.PAN/PANI nanofibers with high electrical conductivity can accelerate the electron transfer and ion diffusion.Abundant nitrogen atoms in PPCA act as active sites to inhibit the shuttling effect of polysulfides in the electrolyte.The lithium-sulfur battery assembled with PPCA as sulfur host delivers a first-cycle discharge specific capacity of 1027 m Ah g^-1at 0.1 C,and the reversible specific capacity is remained at 400 m Ah g^-1after 400 cycles.2.PAN/PANI/graphene aerogel（PPGA）composite materials with high electrical conductivity were synthesized by electrospinning combined with hydrothermal method.PAN/PANI nanofibers have good compatibility with the r GO sheets and can be easily embedded into the sheets.This special structure reduces the density of the aerogel and increases its elasticity.With high specific surface area of 257.527 m²g^-1,PPGA provides enough space to achieve high sulfur loading and also is conducive to the infiltration of electrolyte,and thus the electrochemical reaction can be fully carried out.Abundant nitrogen atoms in PPGA can effectively anchor polysulfides and improve the electrochemical performances of lithium-sulfur battery.At a current density of 0.5 C,the specific discharge capacity is 412 m Ah g^-1after 500 cycles,and the average decay rate of each cycle is 0.096%.Moreover,the composite electrode exhibits excellent good rate performance.3.Ti O₂/V₂O₅hollow nanofibers were fabricated by electrospinning technology.The hollow structure provides sufficient internal space for the volume change of sulfur.By the aid of the rapid catalytic effect of Ti O₂on polysulfides and the excellent adsorption capacity of V₂O₅on polysulfides,the composite electrode has an initial discharge specific capacity of 865 m Ah g^-1at 0.5 C,maintains a reversible capacity of 481 m Ah g^-1after 800cycles,and the capacity decay rate is only 0.055%per cycle,demonstrating excellent cycle stability.4.Ni Co₂O₄@PPy flower-like composite materials were synthesized via combination of hydrothermal method,high calcination and in-situ vaper phase polymerization.Ni Co₂O₄is conducive to absorbing and following catalytic conversion of polar intermediates.PPy with excellent conductivity can increase the electron transfer,concurrently promote lithium ions diffusion.Besides,PPy can provide abundant active sites to anchor polysulfides to efficaciously confine polysulfides shuttling.Ni Co₂O₄@PPy is simultaneously served as the effective cathode material and multifunctional interlayer for anchoring-catalyzing polysulfides,which is conducive to accelerating the conversion of polysulfide to Li₂S₂/Li₂S,reducing polarization and accelerating electrochemical kinetics,and delivering a satisfactory initial discharge capacity of 1588 m Ah g^-1at 0.1 C.The excellent capacity of423 m Ah g^-1is maintained at 2 C after 400 cycles.