Study On The Metal Oxide/Graphene And Sulfur-carbon Composite Of Second Lithium Batteries

Metal oxides are considered as the potential anode materials due to their muchhigher theratical specific capacity than that of graphite. However, these metal oxidestend to encounter large volume expantion during Li ion insertion/extraction process,ledding to the unfavorable factors of material pulverization and electrode crack thatrestrain their cycle performance. Graphene was used in this work as carrier to holdSnO2and MoO2nanoparticles due to its large surface area, high electric conductivityand super mechanical stability.Compared with lithium ion battery, lithium sulfur battery could deliever highertheoretical energy density. In addition, sulfur has the advantages of nature abundance,environment friendly and low cost. However, low utilization and worse cyclability aresignificant drawbacks that challenging this new battery. In order to address thesedeficiencies, nafion (Perfluorinated sulfonic acid) coated S/C composite and nafioncoated electrode were prepared through two different methods.SnO2/graphene composite was prepared by a simple sol-gel method using SnCl2and graphene as raw materials. It is observed from SEM and TEM that SnO2nanoparticles are homogeneously attached on the surface of graphene sheets.Compared with pure SnO2particle, SnO2/graphene composite shows a much highercyclability. The composite could achieves726.6mAh·g-1after20cycles at the cutterndensity of100mA·g-1.MoO2/graphene composite was synthesized from graphite oxide and ammoniummolybdate by using one-step hydrothermally method. The result suggests the productcan exhibit the best cycle performance at condition of adding graphene50mg andammonium molybdate2.3g. After60cycle, the discharge specific capacity were1037mAh·g-1. MoO2and graphene could form the covalent bonds in the process ofhydrothermal, so the MoO2nanoparticles were firmly anchoring in the surface ofgraphene. The stable structure of graphene can restrict the volume change of MoO2,which provided the buffer space for volume expansion, thereby prevent the electrodepieces and improved the cycle performance of battery.Nafion coated S/C composite with different nafion content was prepared in thiswork. The nafion coating is proved to be helpful in increasing the cyclability andcoulombic efficiency of S/C composite. In a series of experiments, the S/C compositewith5%nafion content shows lowest resistivity and hightes cycle performance. Thecoated S/C composite deliever an initial discharge capacity of1505.4mAh·g-1and retains740.2mAh·g-1after120cycles at0.2C. Moreover, the coated S/C compositeachieve629mAh·g-1after100cycles at high rate of0.5C. Coating a nafion film onelectrode was also exmined for lithium sulfur battery, and the result shows that nafionfilm could effectively improve the coulombic efficiency of S/C electrode bysuppressing shuttle effect. In a LiNO3free electrolyte, coated electrode shows anincreasement of efficiency by about40%compared with uncoated electrode. Besides,nafion can also be served as binder, which would strength the stalibility of theeclctrode and thus prolong its cycle performance. The coated electrode could delieveran initial discharge capacity of1084mAh·g-1and retains879mAh·g-1after100cyclesat0.2C.Furthermore, S/C composite was prabricated by electrochemical depositingmethod using carbon felt and Bp2000as suffur carrier. S/carbon felt composite withdifferent sulfur content was obtained by maintaining charging current and changingthe polarization time. The S/carbon felt composite that after8hours depositating wasselected to be tested electrochemical properties, and the result show that thiscomposite deliever an initial discharge capacity of1060.6mAh·g-1retains669.8mAh·g-1after80cycles at0.2C. Whereas, the first discharge capacity of S/Bp2000composite that after10hours deposition could reach1473.8mAh·g-1with a highutilization of89%.