Synthesis And Performance Study Of Graphene Based Cathode Materials For Lithium-Sulfur Battery

Lithium-sulfur battery is the most promising candidate of the next generation energy storage system due to its high energy density,low-cost and environment friendly.Aiming at the issue such as low utilization of active material and poor cycle performance,the design of three-dimensional graphene with specific morphology and surface modification was carried out.The pyrrole induced N-doped graphene?NGE?host was synthesised.The graphene sheets presente a three-dimensional interconnected structure and the active material is distributed in the graphene pore structure.The NGE-S electrode exhibited higher electrochemical activity and cyclic stability than that of the graphene?GE?prepared by direct heat treatment.The reserved capacity was improved 252 m Ah g^-1,corresponding to an increased capacity retention of 26%when compared with the GE-S electrode after 300 cycles.Calcination temperatures have significant impact on the physical properties of graphene hosts.The contrast experiments show that the graphene host calcined in 850?has the best electrochemical properties.The ratio of graphene oxide?GO?to pyrrole in raw materials mainly affect the amount of nitrogen doping in the graphene.The results shows that the best electrochemical properties are obtained when the additive amount of GO was 200 mg.The initiator for the polymerization of pyrrole was employed as the graphene layer etching agent at the same time.The three-dimensional N-doped graphene nanomesh foam as sulfur host was synthesised by hydrothermal method followed by calcination treatment.The prepared graphene with a typical three-dimensional structure and the graphene sheet surface was etched with nanoholes with diameters about 30 to 70 nm.The surface area was more than twice of 3DGE.The capacity decay of 3DGE-S electrode was 34%after 0.5 C for 200 cycles.In contrast,the capacity decay of 3DNGF-S electrode was only 16%?The reserved capacity was improved 39%at 2 C than two-dimensional graphene electrode?VGE-S?.After 500 cycles,the capacity retention was 70%,corresponding to a capacity decay of 0.06%per cycle.The three-dimensional N-doped graphene?NTGE?was prepared by urea assisted thermal reduction of GO in polyurethane sponge template.The specific surface areas was improved by over eighteen times than urea-drived graphene?UGE?.The stack of graphene sheets in calcination process was reduce effectively due to the exist of template.When compared with the graphene without nitrogen-doping?TGE?,the capacity retention improved 41%after 0.5 C for 300 cycles.The three-dimensional N-doped graphene with hierarchical pore strcture?NPGE?was prepared by introduced KOH as pore-forming agent.The best electrochemical properties are obtained when the ratio of GO to KOH is 1:4.After0.5C for 600 cycles,the capacity retention as high as 88%correspond to a capacity decay of 0.02%per cycle.The three-dimensionalN-dopedgraphenesupportmetaloxides?Ni Fe₂O₄/3DNGE?was synthesized for sulfur host in Lithium-sulfur battery by one-step hydrothermal method.The electrical conductivity was improved due to the three-dimensional graphene skeleton.The active material utilization was as high as90.6%at 0.2 C under the sulfur content of 70%.After 1C for 700 cycles,the capacity retention was 79%,corresponding to a capacity decay of 0.03%per cycle.By optimizing the ratio of metal oxides and graphene,the best performance was obtained when the metal oxides content was 16.7%.The visual charge-discharge test indicates that the Ni Fe₂O₄ nanoparticles is the key for the stable cycle process.The polar metal oxides are in favour of absorbing polysulfides than non-polar carbon materials.