Functional Carbon Materials:Design,Preparation And Their Application For Lithium-Sulfur Batteries

Considering its remarkable special capacity(1675 mAh g-1)and high energy density(2500 wh kg-1),lithium sulfur batteries have the ability to meet the urgent demand for exploiting a new generation of energy storage system.In addition,as the active materials of cathode,the sulfur have lots of advantages,such as natural abundance and non-toxic.As the result,Li-S batteries have attracted more interest and been considered as one of the most popular candidates for new power sources.Unfortunately,up to now,the practical application of Li-S batteries are still hindered by some unsolved issues,including that insulating nature of sulfur and lithium sulfides,the "shuttle effect" occurred between cathode and lithium anode and enormous volume change of sulfur electrode.Undoubtedly,these troublesome problems lead to the poor utilization of sulfur and short cycle life of Li-S batteries.To solve these tough problems,we focus our attentions on the design of sulfur cathode though preparing several kinds of functional carbon materials with different structure or elemental compositions,and used them as scaffolds of sulfur.The corresponding C/S composites are worked as cathode materials and their electrochemical performance were systematic investigated in this paper.The main works are summarized as follows:(1)A kind of hierarchical micro/mesoporous carbonaceous nanotubes(HMMCNT)are synthesized by high-temperature calcination of Te/C nanocable at Ar atmosphere,benefited from its thick microporous wall and the inner hollow cavity,it was used as scaffold for Li-S batteries.It has been proved that there exist a portion of small sulfur molecules(S2-4)in the microporous of HMMCNT via BET and XPS analysis and the related electrochemical performance of HMMCNT-S-50 were measured between different voltage windows.The results showed that the better capacity of HMMCNT-S-50 can be obtained by optimizing the discharge cut-off voltage under the condition that S8,S2-4 molecules and LiNO3 additives are coexisted in the electrode.Furthermore,the electrochemical performances of HMMCNT-S composites with sulfur loading of 40%and 50%were measured and analyzed at the voltage window of 1.4-2.8 V.(2)A nitrogen and phosphorus dual-doped porous carbon nanotube(NPPCNT)as sulfur scaffold were prepared by mixing of ammonium dihydrogen phosphate with Te/C nanocables which have been wiped off Te wire in advance and heating in the following step.The nitrogen and phosphorus elements were permeated into the carbon framework during calcination in Ar.It was shown that NPPCNT has the strong affinity for lithium polysulfide via the adsorption test and analysis of XPS spectrums.Furthermore,a kind of porous carbontube(PCNT)were also synthesized using the same preparation procedure except for adding ammonium dihydrogen phosphate.After sulfur infiltration,the electrochemical performance of NPPCNT/S-60 and PCNT/S-60 electrodes were measured and the advantages and disadvantages of NPPCNT as sulfur holder were initially discussed depending on their capacity values.(3)A composite material that CoCO3 nanoparticles were in situ anchored on the porous carbon nanotube(PCNT@CoCO3)were synthesized via hydrothermal reaction.When it was used as sulfur matrix,the related PCNT@CoCO3/S-60 electrode showed excellent cycling stability.What was more,when it was used as a coating layer on the commercial separators and combined with NPPCNT/S-60 electrode to assemble the cell,the corresponding discharge capacity after 150 cycles could still maintain 889 mAh g-1 at the current density of 500 mA g-1.Compared with the electrochemical performance of cell which used separator with PCNT coating layer,the functions and shortcoming of CoCO3 nanoparticle in electrode for promoting the electrochemical performance of Li-S batteries were preliminary explored.(4)From the point of industrialized application,a class of nitrogen and oxygen dual-doped nonporous carbon materials(NONPCMs)as the sulfur host are synthesized using the glocuse and ployvinylpyrrolidone(PVP)as raw materials just by a simple hydrothermal reaction and high-temperature calcination process.The related NONPCM/S composites with different sulfur loading showed excellent cycling stability.What is more,it has been objectively proved the NONPCMs have the ability to anchor the lithium polysulfides via solely chemical interactions between lithium polysulfides and electronegative heteroatoms though theoretical calculation and adsorption test.In addition,the NONPCM precursor in the kilogram scale(NONPCM-K)were successfully obtained and the corresponding NONPCM-K/S electrode also showed remarkable cycling performance,when the sulfur loading was 70%,the discharge special capacity could maintain 540 mAh g-1 after 300 cycles at 1.6 A g-1...