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Structural Design And Interfacial Engineering Of High Performance Graphene Based Lithium Battery Materials

Posted on:2017-10-02Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y F DonFull Text:PDF
GTID:1311330488452288Subject:Functional Materials Chemistry and Chemical Engineering
Abstract/Summary:PDF Full Text Request
Due to the advantages of high energy density, high power density and no memory effect, lithium ion batteries (LIBs) have drawn much attention, and have be applied in portable electronic devices. However, present commercial electrode materials can not deliver sufficient energy to realize longe range and fast charge and discharge cycles for electric vehicles. Therefore, electrode materials with high performance characteristics are of great significance in terms of scientific reseach and practical applications. In this thesis, electrochemical active materials with high theoretical specific capacity (metal oxide or sulfur) and graphene nanosheets with excellent physical and chemical properties were employed to construct high performance electrode materials, mainly addressing the poor structure stability of metal oxide anodes and shuttle effect of sufur cathodes, and the applied scientific concept is that both nanotructural design and interfacial engineering should be optimized to construct high performance electrodes. Several high performance lithium electrodes were developed in this thesis, such as compressible graphene/CoO aerogels as binder-free electrodes, metal oxide hollow nanostructures strongly coupled with graphene nanosheets, graphene/SnO2/polyaniline sandwiched anode and functionlized graphene/sulfur ccathode, graphene/Sulfur/polypyrrole sandwiched cathode. The detailed research contents and results are highlighted as follows:Synthesis of compressible graphene/CoO nanostructures areagels as binder free electrode materials.Ultilizing graphene oxide (GO) nanosheets as precursor and pyrrole as auxiliary agent, graphene aerogels supported with CoO nanowires (NWs) or urchin-like nanospheres were successfully prepared mainly via hydrothermal assembly-freeze drying-annealing routes. Both graphene aerogels/CoO hybrids could be directly used as binder-free anodes in LIBs, and exhibit high capacity (over 550 mAh g-1) after 100 cycling times, superior that of conventional CoO powder anode. CoO NWs were uniformly loaded on graphene aerogel with strong coupled interfaces greatly improve electrochemical performance.Construction of metal oxide hollow nanostructures strongly coupled with graphene nanosheets for high performance lithium ion batteries. Based on the scientific theories, such as crystal growth control, carbothermal reduction and Kirkendall effect, various metal oxide hollow nanostructures binded on graphene nanosheets by graphitic carbon layers (h-MO@C@G), such as h-Fe2O3@C@G, h-Co3O4@C@G, h-NiOx@C@G, were generally synthesized mainly via metal nitrate-polyvinylpyrrolidone (PVP) coating on GO nanosheets, high temperature anealing and subsequent mild oxidation. The particle size of h-MO, the macroporous structure of graphene nanosheets as well as the MO content in the h-MO@C@G hybrid could be precisely controlled by PVP dose. Benefited from the synergistic effect of h-MO and the strongly composite interfaces, the resulting h-MO@C@G all outperform high rate capacity ability up 300 mAh g"1 at 15 A g-1.Construction of graphene/SnO2polyaniline sandwiched anode. SnO2 nanoparticles are firstly anchored on graphene nanosheets via in situ reduction of GO by Sn2+ ions, and then intimately coated with Pani with the help of phytic acid (PA). The nanoparticles with nearly uniform sizes are spatially separated with each other, and the thickness of Pani coating layer could be precisely controlled. The resulting sandwich-like hybrids would facilitate the diffusion for electrolyte into the interior of the electrode, reduce the transport distance of ions, and the conductivity and stablility of SnO2 nanoparticles sandwiched between graphene and conductive polymer were greatly improved, furthermore, the Pani coating layer facilitates the formation of stable SEI films, therefore, the final DF-SnO2/G@Pani anodes exhibit high capacity of 700 mAh g-1 (approach to theoretical capacity of SnO2) at current density of 200 mA g-1, high rate capacity ability up to 200 mAh g-1 at current density of 2000 mA g"1, and long cycle life up to 700 times.Functionalized graphene were employed for high performance lithium sulfur batteries. Ethylenediamine (EDA) functionalized graphene nanosheets (EFG) was firstly synthesized by low temperature hydrothermal method and employed for sulfur loading, resulting in EFG-S hybrids. Both density functional theory (DFT) calculations and experimental results indicated EFG could strongly interact with both sulfur and its discharged product (Li2S). Moreover, graphene with good mechanical properties would be good for the structure stabilityof EFG-S cathode. As a result, EFG-S cathode could deliver high capacity of 650 mAh g-1 after 350 cycles with 80% capacity retention.Construction of graphene based mesoporous carbon/sulfur/polypyrrole sandwiched cathode (GCS@PPy). Ultilizing graphene oxide (GO) nanosheets as precursor, sulfur was successfully sandwiched between graphene based mesoporous cabon nanosheets and PPy coating layer mainly via nanocasting, sulfur melting-diffusion and PPy coating processes. Sanwich-like GCS@PPy can efficiently improve the sulfur utilization and electrolyte diffusion in electrodes, meanwhile, the presence of graphene, mesoporous carbon in combination with conductiong polymer could enhance the conductivity of sulfur, and accommodate the volume expansion of sulfur during cycling, resulting in stable sulfur based cathode. Therefore, the GCS@PPy electrode exhibit a high reversible capacity of 935 mAh g-1, an ultra slow decay rate of 0.05% per cycle over 400 cycles, and high rate capacity ability up to 430 mAh g-1 at current density of 4 C.
Keywords/Search Tags:graphene nanosheets, metal oxide, sulfur, lithium ion batteries, lithium sulfur batteries
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