Preparation And Lithium Storage Test Of Co₃O₄and Si Based Composites

During the past few decades, scientists from all over the world are energeticallysearching for new green energy sources and/or energy storage materials due to theenergy crisis and the concomitant environmental problems, all of which are inducedby the rapid society development. The rechargeable lithium ion battery, which isdeveloped in the20th century, has been widely used in the portable phones, portablecomputers, and cameras and so on owning to its lots of outstanding merits such as thehigher capacity, the higher voltage and the long life. Recently, most of the traditionalbatteries have been substituted by the rechargeable lithium ion batteries. In addition,high capacity rechargeable lithium ion batteries have been used in the electro-car andare expected to be the main energy source of the car in21st century. Among theelectrode of the lithium ion battery, the transitional metal oxide, cobalt oxide, forexample, have attracted wide research interests because of its high theoretical capacity.Also, owning to the highest known theoretical charge capacity of4009mAh g-1,silicon has received focus from researchers all over the world. In this graduationthesis, we have conducted the fabrication, characterization and the electrochemicaltest of two cobalt oxide based and one silicon based composites. We have devotedourselves to find the highest output of cobalt oxide and silicon. The main content isconcisely described as follows:Firstly, we have employed the crystallized silicon as the substrate and the metalassisted chemical etching method to fabricate silicon nanowires. And then we havecharacterized the fabricated silicon nanowires, finding that the diameter is about30-120nm, the length is about30um and the growth orientation is in good agreementwith the silicon substrate. So far, we have successfully fabricated large-area,highly-oriented1D silicon nanowires （Si NWs） arrays. The second keystone of ourelectrode preparation process deals with coating the Si NWs with Co nanoparticlesand subsequent annealing in the air to acquire Co₃O₄. Again, electroless metaldeposition （EMD） emerges as a powerful technique as it also allows the directgrafting of the Co nanoparticles onto the Si NWs. In the last, we have characterizedthe composites with SEM, TEM and XRD. The resulting Co₃O₄/SiNWs compositewas evaluated as electrodes in Li half cells which were cycled in a galvanostatic modeat different rates. Control experiments were also made, in which the electrode is madefrom commercially available cobalt oxide. We believe the high capacity retention andrate capability of the Co₃O₄/SiNWs electrode come from its unique configuration.Secondly, we have prepared the GO by employing the modified Hummersmethod, which is consisted of mainly three steps: the high temperature, the lowtemperature and the medium temperature. In the second place, we have uniformlymixed the prepared GO with the commercially available cobalt oxide. In the thirdplace, a piece of commercially available nickel foam （NF） is subsequently coated withCo₃O₄/GO mixture using a facile “Dipping and Drying” process. The process isrepeated several times to make sure that NF have been fully packed with Co₃O₄/GO mixture. In the last, after appropriate cycles of “Dipping and Drying”, the compositewas treated in a microwave oven to efficiently reduce GO, forming the ultimatecomposite Co₃O₄/Graphene. After the preparation, we have characterized thecomposite with Raman, energy dispersive spectroscopy （EDS） and SEM.Subsequently, our attention was devoted to the functional investigation of theobtained composite. Not only the cyclic performance was tested, charge-dischargecurves at different current densities were also measured considering that the ratecapability is also very important for practical applications. The results show that theenhanced hierarchical double porous Co₃O₄/Graphene architecture composite displayshighly reversible capacity, excellent cyclic performance and high Coulombicefficiency with good rate capability.Thirdly, we have prepared the free-standing Si/G alternative stratum structurecomposite. GO was synthesized by oxidizing graphite with the modified Hummersmethod and the Si nanoparticles were purchased from Alfred company. The dried GOand purchased Si nanoparticles were carefully scaled and then disperse them intode-ionized （DI） water to form a uniform solution, respectively. To form the Si/Galternative stratum structure, part of the suspension of GO and Si of appropriateweight was vacuum filtered. And then such GO and Si filtering-transferring processwas repeated several times to prepare five-layered or nine-layered Si/graphene oxidestructure. After the preparation, the composite was peeled off from the membrane andwas immersed into HBr to effectively reduce the GO. In the last, the as-preparedfree-standing flexible alternative stratum structure composite was directly evaluatedas anodes for lithium ion battery and the batteries based on this new structureexhibited a large capacity as well as improved cycling performance.