The Preparation Of Graphene Oxide/Sulfur Composite With Different Sulfur Source And Its Electrochemical Performance Study

Lithium ion battery is a kind of high-capacity energy storagesystem with good development prospect. Elemental sulfur existswidely in nature and it is friendly to environment and with low cost.But lithium sulfur batteries still face these prominent problems thatthe insulation of sulfur results in low utilization rate of activematerial and high solubility of polysulphide in the electrolyte in theprocess of charging and discharging causes bad cycle performance.In this paper, by changing sulfur source, we made small moleculesulfur and conductive material graphene oxide compound to try toimprove the conductivity of the electrode material and improvebattery performance.(1) Graphite oxide was prepared by improved Hummers method.Its physical properties have been characterized by FT-IR, XRD, SEMand the results showed that graphite was oxidated more thoroughlyand graphite layer spacing increased obviously after oxidation(0.739nm), a large amount of oxygen-contained functional groups were inserted between the layers. Graphene oxide solution wasachieved under ultrasound as lithium-sulfur battery cathodemodified material. The composite were prepared by using Na2S2O3as sulfur source to react with hydrochloric acid and compound withgraphene oxide with in-situ deposition route and the traditionalroute.The electrochemical performance of composites werecompared. Under50mA/g current density, initial discharge specificcapacity can reach589mA h/g, the values of traditional methodscomposite materials and pure sulfur as the positive wererespectively429mA h/g and385mA h/g; The specific capacity ofliquid phase in-situ composite was also the highest after20cycles.According to the results of electrochemical impedance test, in-situcomposite had minimum charge transfer resistance and addedvalue of its positive charge transfer impedance was relatively smallafter20cycles, reaction kinetics was superior to others.(2) Using Na2Sxas sulfur source to react with HCOOH, grapheneoxide was added with in-situ deposition route to preparecomposites. Change Na2Sxconcentration, the physical propertiesand electrochemical performance changes of the composites werestudied with sulfur content changing. Thermogravimetric analysisshowed sulfur content measured were49.4%,69.5%and74.4%. Thefirst discharge specific capacity were329.1mA h/g,524mA h/g and 498.3mA h/g; After20cycles, the second composite gave amaximum reversible specific capacity of240mA h/g, capacityretention rate was45.8%. And from its2nd,3rd,5th charging curve,the battery can reach full charged state, typical polarizationphenomenon of lithium sulfur battery got inhibited. Electrochemicalimpedance results showed that charge transfer impedance of thecomposite2was small and lithium ion diffusion degree was high.(3) Sublimed sulfur was soluble in CS2, which was mixed withgraphene oxide, composite materials with different sulfur contentwere prepared. Another composite was prepared with sublimedsulfur directly adding to the graphene oxide solution.Compare thetwo physical and electrochemical properties. XRD, FT-IR, SEM testshowed that the former was more evenly with smaller particle sizeof sulfur, part of graphene oxide was reduced by sulfur. XRD resultsshowed the characteristic diffraction peak of graphene, and SEMspectrum can see clearly the distribution of graphene sheet,especially for the composite2with suitable sulphur content,graphene, graphene oxide and small molecule sulfur evenlydistributed. The infrared spectra showed that a new C-S bondappeared. Electrochemical testing results showed that the firstdischarge specific capacity for GO/S (CS2) composite material1,2,3were respectively398mA h/g,510mA h/g,467mA h/g at50mA/g. The reversible capacity retention rate of GO/S (CS2) compositematerial1,2,3and GO/sublimed sulfur composites were24.1%,28.4%,22.6%and13.8%after20cycles. Obviously, the value of thecomposite2was the highest, the electrochemical performance wasexcellent.