Preparatio Of Sulfur/Conductive Polymer Compoite Electrode Materials And The Modification Studies

Element sulfur(S)has developed to one of the most promising cathode materials for next-generation batteries in recent years because of its owning many advantages such as high energy density,environmental friendliness,and lower cost.As is known,the redox reaction in Li-S batteries can be written as 16Li +S8 =8Li2S,in which the sulfur cathode can deliver a theoretic specific capacity of 1672 mAh g-1(2600 Wh kg-1).The Li-S batteries show two voltage plateaus at 1.9-2.1 V and 2.4V(vs.Li+/Li)in the discharge curves when operating.However,the practical use of sulfur cathode is limited by some fatal defaults including the isolated nature of sulfur,large volume change during lithiation and delithiation,dissolution of the intermediate products polysulfides(Li2Sn,4 ? n ? 8)and the resultant shuttling effect.These defaults lead to a serious capacity degradation and utilization efficiency of the active materials.To resolve these problems,we decided to carry out the following work:(1)producing the nanoscale sulfur particles,(2)combining the sulfur nanoparticles with conductive polymer,(3)capsulating the sulfur nanoparticles by graphene and combining with conductive polymer,(4)introducing metal chalcogenides to the composite materials.(1)Sulfur particles with a diameter of 120-130 nm were prepared by liquid-phase precipitation method.They are modified by various kinds of acids,and the conductive polymer is obtained by in-situ polymerization of aniline.We studied the effect of the acid doping on the morphology,structure and electrochemical performance of the as-prepared samples.It is found that the sulfur displays orthometric structure of ?-S8,and maintains this structure after modifying by acids and conductive polymer.Among the acids,polymer chains doped with phytic acid(PA)are three-dimensional networks,and thus forming a porous structure.The S/PANi-PA sample presents the best electrochemical performance among the various acid doping samples.It can deliver an initial capacity of 1278 mAh g-1,and remains 75.6%of the initial capacity after 50 cycles when oprated at 0.1C rate over the voltage range of 1.5-3.0 V.At the rates of 0.2C and 0.5C,the maximum capacities are 1126 and 934 mAh g-1,respectively.The corresponding capacities still retain 815 and 763 mAh g-1 after 50 cycles.(2)Effect of the graphene oxide that prepared by Hummers on the electrochemical properties of the doped S/PAN-PA composite materials is studied.Viewing from the SEM/TEM images,sulfur nanoparticles are encapsuled by the graphene layer,and connected each other by PAN nanowire.The results indicate that the obtained S@rGO/PANi-1.5 sample has the best electrochemical preparties when the concentration of doping graphene oxide is 1.5 mg ml-1.It can deliver the capacities of 1375,1252,1105,929 and 807 mAh g-1 at the rates of 0.05C,0.1C,0.2C,0.5C and 1C in the first cycle,respectively.The retention ratios of capacity are 77.2%,73.2%and 72.7%after 50 cycles at the rates of 0.1C,0.2C and 0.5C,respectively.The coulombic efficiency of this sample reaches more than 96%during cycling.The results suggest that the electronic conductivity is enhanced by the inter-connection of graphene with the conductive polymer chains,and the volume expansion of sulfur particles is affectively alleviated by the graphene encapsulation,resulting in an improved electrochemical performance.(3)CoS2/rGO composite material were prepared by a hydrothermal method,and were combined with the sulfur by a melton-diffusion approach.The resultant S@CoS2/rGO sample shows a porous structure,which facilitates penetrating of electrolyte into the pores to form three-dimensional channels for ion transportation.The BET specific surface area is 39.4 mg2 g-1.From the primary results of electrochemical tests,the sample can deliver the charge/discharge capacities of 1766/1684 mAh g-1,1526/1468 mAh g-1,1356/1270 mAh g-1 and 1160/1127 mAh g-1 in the 1st,5th,20th and 50th cycles,respectively.The potential plateaus in the charge-discharge curves agree well with those of the redox peak in the cyclic voltammograms.The results indicate that it is worth studing the effect of metal sulfides on the electrochemical performance of the sulfur cathode further.