Preparation And Properties Of Honeycomb Porous Carbon Nanofiber Cathode Materials And Aramid Membranes For Lithium-sulfur Batteries

As a novel lithium battery,lithium sulfur?Li-S?battery has attracted wide attentions at home and abroad due to the high specific capacity of 1675 mAh g^-1 and the large theoretical energy density of 2600 Wh kg^-1 in theory.But the poor conductivities of sulfur and sulfide,the large volume change during the process of releasing/embedding lithium,the heavy "shuttle effect" of polysulfide?intermediate products in electrochemical reaction?and so on hinder the commercialization of Li-S batteries.The electrospun nanofibers have become research hotspots as cathode and separator material for Li-S batteries due to the high aspect ratio,the high specific surface area,the possible mass production,etc.In this paper,high performance electrospun nanofibers based cathodes and separators for Li-S batteries were developed through the structural bionics and the nanoparticles doping,aiming to reduce the "shuttle effect" of polysulfide and improve the electrochemical performance.The main contents are as follows:?1?Firstly,polyvinyl alcohol/polytetrafluoroethylene?PVA/PTFE?nanofibers were prepared through electro-blown spinning technique using their spinning solution with a certain amount of complexant Boric Acid?BA?.The PVA based honeycomb hierarchical porous carbon nanofibers?PCNFs?were successfully prepared by removing PTFE materials after the preoxidation and carbonization process.Three composite electrodes named CNTs-S,PCNFs-S and PCNFs-CNTs-S were prepared by the sulfur fumigation technology in vacuum.The structure and performance of PCNFs and its composite material were characterized by SEM,TEM,BET,XPS,XRD,Raman and so on.Meanwhile,their electrochemical performance of three electrodes were compared.The results showed that PCNFs had a hierrachically porous structure including micropores,mesopores and macropores,as well as high specific surface area and pore volume.The formation of mesopore and macropore mainly depended on the PTFE nanoparticle,and micropore mainly depended on thermal decomposition of BA.In these composite electrodes,the increased sulfur loading owing to the meso/macropores,confined polysulfide owing to the a meso/micropores,the improved electrical conductivity by adding CNTs made the PCNFs-CNTs-S electrode low impedance and polarization phenomenon,and excellent electrochemical performance.The battery with the cathode presented a high initial discharge specific capacity of 1302.9 mAh g^-1 at 0.5 C and the discharge specific capacity was still kept at 809.1 mAh g^-1 at 0.5 C after 300 cycles(1 C=1675 mAh g^-1).?2?Secondly,the CeF₃-doped PCNFs were prepared using above methods after loading a certain amount of CeCl₃ in the PVA/PTFE solution.The effects of CeF₃ contents and carbonation temperatures on the micromorphology,electrical conductivity and electrochemical properties of the electrode were investigated.The inhibition action of CeF₃ nanoparticles on "shuttle effect" of polysulfide was investigated by Li₂S₆ adsorption experiment.The study showed that CeCl₃ had ignorable effect on the spinning performance when the content of CeCl₃ was less than 10 wt%and CeCl₃ turned into six square structure CeF₃ nanoparticles during the high temperature carbonation.The doped CeF₃ maintained the honeycomb hierarchical porous structure and distribution,but made the conductivity decreased slightly.The doped CeF₃ significantly enhanced the adsorption of PCNFs to Li₂S₆ solution,provided more active sites,effectively inhibited the "shuttle effect" of polysulfide and further enhanced the electrochemical performance of the PCNFs electrode.When the CeF₃ content was 7.5 wt%,the battery with the CeF₃-doped PCNFs electrode exhibied high first-cycle discharge capacity of 1395.0 mAh g^-1 and excellent cycling stability with good capacity retention of 901.2 mAh g^-1 after 500 cycles.And the initial discharge specific capacity of the battery was 1169.1 mAh g^-1 at 2 C rate.?3?The relationship model among pore size and distribution,specific surface area,pore volume,sulfur carrying capacity,test conditions?testing rate and cycle times?and battery electrochemical performance?initial specific capacity and attenuation rate specific capacity?of the battery were established using the spearman rank correlation coefficient and typical correlation coefficient analysis method.The results of the model analysis showed that the attenuation rate specific capacity for battery was greatly affected by the loading sulfur content,testing rate and cycle times.And the initial discharge specific capacity was mainly affected by the percentage of micropore and mesopore in the carbon materials,followed by the significant correlation with the specific surface area in the carbon material and the testing rate.Meanwhile,the amount of sulfur loading has some influence on the initial discharge specific capacity.In addition,the two analysis methods both showed the significant effect of mesoporous on the initial discharge specific capacity of the lithium sulfur battery,confirming the feasibility of applicating the cathode of honeycomb porous carbon nanofibers in Li-S battery.?4?A tree-like fluoride doped PMIA nanofiber membrane?Tree-like F-doped PMIA NM?with excellent high temperature resistance was prepared by electrospinning a spinning system composed of PMIA solution including tetrabutylammonium chloride?TBAC?and hexafluoroacrylate?Oliphobol?.The micro and chemical structures were characterized by SEM,XPS and FTIR.The effect of TBAC content on pore size and mechanical properties of the nanofiber membrane were investigated.And the thermal stability,gelation and electrochemical properties of the membrane were studied.The results showed that the tree-like structure of nanofibers become more notable with the increase of TBAC content from 0 wt%to 5 wt%,result in greatly improved mechanical properties and reduced pore size.The addition of fluorine emulsion significantly improved the electrolyte uptake and gelation phenomenon of PMIA nanofibers.Compared with commercial membranes,the F-doped PMIA and tree-like F-doped PMIA both had better thermal stability below 240 ?.The small pore size and gelation of Tree-like F-doped PMIA can hinder the "shuttle effect" of polysulfide.The battery with the Tree-like F-doped PMIA presented high ionic conductivity and low impedance.The battery with the Tree-like F-doped PMIA?5%TBAC?exhibited high first-cycle discharge capacity of 1222.5 mAh g^-1 and good capacity retention of 745.7 mAh g^-1 after 800 cycles at 0.5 C.?5?Take advantage of the chemical adsorption between metal oxide and polysulfide,a double-layer composite nanofibrous membrane composed of F-doped PMIA and F-Mn₃O₄-co-doped PMIA nanofiber membrane?F-Mn₃O₄-co-doped PMIA DCNM?was prepared successfully by electrospinning technique.The effects of Mn₃O₄ content on the fiber morphology,diameter distribution,surface roughness,membrane strength,and the absorbing and preserving liquid electrolyte were discussed.The electrochemical properties of PMIA,F-doped PMIA and F-Mn₃O₄-co-doped PMIA were compared.The results presented that doping Mn₃O₄ could increase the roughness and mechanical properties of nanofiber membranes.The liquid retention rate of F-Mn₃O₄-co-doped PMIA was up to 1450%and its the ionic conductivity was 2.19×10^-3 S cm^-1,higher than those of PMIA and F-doped PMIA.The battery with the F-Mn₃O₄-co-doped PMIA showed a high initial discharge capacity as high as 1237.1 mAhg^-1,and super stable capacity retention with 814.0 mAh g^-1 after 1000 cycles at 0.5 C.Meanwhile,the battery also showed a high rate performance with high first-cycle discharge capacity of 1069.1 mAh g^-1 and good capacity retention of 61.9%of initial specific capacity after 600 cycles.