Syntheses And Electrochemical Performance Of Sulfur/bio-carbon And Nano-li₂s@ Porous Carbon Hybrids Cathode Materials For Lithium-Sulfur Batteries

Lithium-sulfur batteries(LSBs)offer high energy,and further the sulfur cathodes are of low-cost,abundant and environmentally benign.LSBs promise great potential to be the new-generation secondary batteries.However,LSBs also meet the problems of the poor electronic conductivity and the large volume change of the sulfur cathodes during cycling,and the dissolution of the reaction intermediates of lithium polysulfides in the electrolyte,causing active material losing,which result in poor cyclic stability and low rate capability.Therefore,sulfur cathode materials are commonly coupled with the high electron conductive carbons in order to enhance their electrochemical performance.In the present thesis,based on a systematic review of the developments of the sulfur cathode materials at home and abroad,and given the advantages of sustainable,environment-friendly and low cost of biomass carbon,new-type biomass carbons are considered to be investigated to as sulfur matrices to fabricate sulfur/carbon composite cathode materials.By exploring new synthesis methods and modifying the synthesis parameters,high performance sulfur/carbon composite cathode materials are obtained,and the mechanism of the improvement of the related composite cathode materials are revealed.Furthermore,given that Li2S is an alternative promising high-capacity cathode material for LSBs,while it still suffers from the problems of complicated synthesis routes,high cost and low cyclic stability and rate capability,etc.,low-cost new methods of synthesizing Li2S and high-performance Li2S/C composite cathode materials are developed.The research results are hopefully significant in the further research and development of high-performance lithium-sulfur battery cathode materials.Spherical carbon with macroporous hollow and microporous shells in particle sizes of 4-6 ?m was prepared by a multi-step pyrolysis using a carbon source of originally porous structured amylose.The micorpores in the carbon shell are mainly in sizes of 1-2 nm.The composite of the spherical carbon infiltrated with 48 wt.%sulfur exhibits high electrochemical performance as a cathode material for LSBs.It shows an initial capacity of 1490 mA h/g,corresponding to 88%of its theoretic capacity,and retains capacity of 798 mA h/g after 200 cycles at 0.1 C.It shows capacities of 907?681?487and 226 mA h/g at 0.5,1,3 and 5 C,respectively.The high electrochemical performance of the composite is attributed to the effective confinement of the micropores in the carbon shell to sulfur,which evidently reduces the dissolution of lithium polysulfides in the electrolyte;the coating of carbon to sulfur,which enhances the electron conductivity of the composite;the macroporous hollow structure of the composite,which buffers effectively the volume change of the sulfur cathode during cycling.Carbon sources of sucrose and enteromorpha are pyrolyzed and followed by a further CO2 etching to form porous carbonaceous materials.The porous structures of the carbons are modified by the modification of the etching temperatures and periods,the pore volumes of the carbon materials are increased,which favor the capacity for storing sulfur.It is found that the carbonaceous materials derived from carbonized sucrose further etched at 950 and 1000 ?,respectively,for 30 minutes,and at 900 ?for 60 minutes,can all constrain partial sulfur to small molecule sulfur(S2-4),the latter of which offers high electrochemical performance than macromolecular sulfur.Among them,the S/C composite derived from carbonized sucrose further etched at 950 ? for 30 minutes exhibits better electrochemical performance than the others.Its first discharge capacity at 0.1 C reaches 1360 mA h/g,and a capacity of 570 mA h/g is retained after 100 cycles.For the carbonized enteromorpha,porous carbon with micorpores in size of ca.3.5 nm is obtained after a CO2 etching at 900 ? for 30 minutes.After combining a sulfur content of 71 wt.%,the S/C composite show high electrochemical properties.The inital discharge capacity at 0.1 C achieves 1434 mAh/g and a capacity of 670 mAh/g is maintained after 200 cycles.The capacities are 998,720,460 and 256 mA h/g at 0.5,1,3,5 C,respectively.The present method of fabricating porous carbon from enteromorpha is facile.The obtained carbon has high specific surface area and high pore volume.It is hopefully expected to be applied in the fields of supercapacitors and catalyst carriers,etc.The high value application of the harmful enteromorpha is desired in both protecting the sea environment and obtaining commercial value.A high purity Li2S with submicron scale is synthesized by ball milling by using LiH and sulfur as raw materials.The synthesis method is facile,low cost and environmentally benign,which offers commercial prospects.The as-syntheszied Li2S shows high activation property as a cathode material for LSBs,which only shows a low activation overpotential of 2.6 V vs.Li+/Li,much lower than that of the commercial Li2S(ca.3.5 V),mainly due to the small particle size of Li2S.By combining the as-synthesized Li2S with PAN via milling and further performed by a carbonization of PAN of the mixture,a new structured Li2S/C composite cathode material with nano-sized Li2S particles dispersively embedded in a carbon matrix is obtained.A high-performance nano-Li2S/porous carbon composite cathode material is obtained by the modification of the ratio of Li2S and PAN.The fabrication method is facile.For the Li2S/C composite cathode material with Li2S content up to 74wt.%,there is almost no initial activation overpotential.The composite shows an initial discharge capacity of 971 mA h/g at 0.1 C,and a capacity of 570 mA h/g is maintained after 200 cycles.The capacities are 746,610,434 and 270 mA h/g at 0.5,1,2,and 4 C,respectively.The high overall electrochemical properties of the composite is attributed to high electrochemical activation of the nano Li2S particles,and the high electron conductivity and high lithium-ion transfer rate and the low dissolution of the polysulfides in the electrolyte due to the effective wrapping of the porous carbon to the nano Li2S particles.