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Synthesis Of Aligned Carbon Nanotubes Array Three-dimensional Composites And Lithium Storage Properies

Posted on:2016-02-10Degree:DoctorType:Dissertation
Country:ChinaCandidate:W N DengFull Text:PDF
GTID:1221330488969544Subject:Materials Science and Engineering
Abstract/Summary:PDF Full Text Request
As a new generation environment friendly green battery, Lithium ion battery has a series of considerable specific advantages, such as high energy density, long cycle life, no memory effect, no pollutionand so on. It is widely used in a variety of portable electronic devices,and is further extended to the application of electric vehicles and hybridelectricvehicle. However, the performance of lithium ion batteries can no longer satisfy the ever increasing demands for high energy density even though they almost reach the theoretical limit both in mass and volume. Therefore the improvement of the research for lithium ion batteries is urgent. Much effort has been invested in the study of CNTs and theresults demonstrate that the properties of mechanics, optics, electrics and chemistryare superior for CNTs. Because of its higher specific surface area and excellent electrical conductivity, CNTs revealenormous potential value in applications fields.Especially, for the energy sciencebased on hydrogen and lithium storage, the performances in adsorption, catalysis andstorage are outstanding so that CNTs have been regarded as a proming electrode matrix materials. In this paper, we have fabricated nitrogen doped aligned carbon nanotubes array(NACNTs), NACNTs/NiO composites, NACNT-SnS2 scomposites, and NACNTs/S composites, which were researched their lithium storage properties. The main results and new findings in this work are summarized as follows:Open-ended NACNTs with tunable morphology and alignment have been synthesized by changing the xylene/cyclohexylamine precursor ratio. We found that the cyclohexylamine fraction plays a key role in the synthesis of NACNTs, affecting both structure and morphology of the NACNTs. According to TEM and SEM analysis, increasing the cyclohexylamine content of the precursor to an optimal value of 35% increased the alignment and diameter uniformity. Furthermore, based on the experimental phenomena and results, a growth mechanism is p roposed, which may be useful for understanding the formation of NACNTs. The controllable synthesis of aligned NACNTs with modulated morphology lays the foundation for the application of these materials in various nanodevices, such as fuel cells and Li-ion batteries.NACNTs/NiO composites with NACNTs contents of 57.1%,47.5%,31.9% and 26.5% have been prepared with the help of both the ethanol-assisted infiltration and the nebulization. Scanning electron microscopy and transmission electron microscopy observations confirmed that an increase in nebulization times results in a increase in the number of NiO anchoring on the surface of the NACNTs. The effect of the NiO loading was also investigate. The synergy effects of NiO and NACNTs lead to the composites exhibing high specific capacity(620 mAh g-1 at 0.1 C) and good cycle stability(600 mAh g-1 at 0.5 C after 200 cycles).The CNTs-SnS2 composites and high-quality 3D structure-based NACNTs-SnS2 composites have been successfully synthesized via hydrothermal method and spray method for NACNTs with the help of nebulization. Well defined regular pore structure combined with well-directed 1D conductive electron paths of NACNTs provided the rapid pathways for ionic and electronic transport, and the appropriate inter-tube space of NACNTs buffer the volume expansion/contraction of SnS2 during cycling. The synergy effects from such a design of the NACNTs-SnS2 electrode lead to its high capacity(645 mAh g-1 at 50 mA g-1 after 7st cycle), good cycle stability(551 mAh g-1 at 100 mA g-1after 100 cycles), and excellent rate capability(287 mAh g-1 at 1000 mA g-1).Hierarchically porous nitrogen-doped aligned carbon nanotubes(HPNACNTs) with abundant accessible micropores and mesopores, well-directed 1D conductive electron paths have been successfully synthesized. Such unique HPNACNTs materials are used as a conducting matrix to encapsulate sulfur as a cathode for Li-Sbatteries. The mesopores within hierarchically pore structure work as the tunnel for better electrolyte diffusion and fast ion transport while the micropores provide room for sulfur loading and prevent the dissolution of the polysulfides. The nitrogen-doping are more effective in forming Sx Li…N coordination-like interactions via the N lone-pair electrons, which coule result in alleviating dissolution and shuttling of lithium polysulfides in the electrolyte. In terms of theseadvantages, the obtained HPNACNTs–S electrode exhibies high specific capacity(1340 mAh g-1 at 0.1 C), excellent rate performance(817 mAh g-1 at 5 C), good cycle stability(979 mAh g-1 at 0.2 C after 200 cycles). This work demonstrates that HPNACNTs with abundant micropores and mesopores are a promising scaffold for encapsulating sulfur to develop high energy/power density Li-S batteries.
Keywords/Search Tags:aligned carbon nanotubes array, NiO, SnS2, KOH activation, Lithium ion batteries, lithium-sulfur battery
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