Preparation,Characterization And Electrochemical Properties Of 3D Graphene Aerogel-based Composites Doped With Silica Or Iron Oxides

As the most promising renewable power source, lithium-ion batteries（LIBs）have been extensive used in the fields of portable electronics and energy storage. It is known that silica and iron oxides, which have the advantages such as low-cost and high specific capacity(500-1000 m Ah g^-1), may be chosen as the prefect negative materials for LIBs. However, the poor conductivity and the collapse or damage during the charge-discharge process of these bulk materials limit their widely applications. On the other hand, as a new material, graphene aerogel（GA） has the advantages of both graphene and aerogel, such as ultra-high specific surface area,high conductivity, low density, excellent electrochemical properties and controllable structure. Meanwhile, the abundant polar functional groups such as carboxyl groups in GA and the good embedded chemical properties make it is easy to integrate with some oxides to form composite materials. In this thesis, nitrogen, silica and several iron oxides doped GA were assembled by a directly hydrothermal method followed by freeze-drying and calcination. The primary work was as follows:（1） Graphene oxide（GO） prepared with a modified Hummers’ method from graphite powders was reduced to graphene hydrogel（GH） via the hydrothermal reduction procedure. Furthermore, it was hydrothermal-treated with ammonia water and freeze-drying to get nitrogen-doped GA（N-GA）. The results showed that the nitrogen was successfully doped into GA. Meanwhile, N-GA maintained the 3D characteristics of GA with cylindrical shape and had large specific surface area(SBET= 379 m² g^-1). After 100 cycles at a current density of 100 m A g^-1, N-GA still showed a high specific capacity of 535 m Ah g^-1. The results indicated that the doping of nitrogen provided more lithium storage active sites than that of the pristine GA, thus improved the electrochemical properties.（2） 3D Si O₂@graphene aerogel（Si O₂@GA） composites were successfully prepared without any structural-directing agent. The results showed that that Si O₂ was in amorphous phase in Si O₂@GA. Furthermore, the composites exhibited meso-macoporous structural characteristic and presented large surface area(SBET =396 m² g^-1) and high pore volume(Vp = 0.672 cm3 g^-1). At a current density of 500 m A g^-1, the Si O₂@GA composites still showed a high specific capacity of 300 m Ah g^-1.（3） The Fe₂O₃ nanoparticles anchored GA hybrid composites（Fe₂O₃@GA） were assembled from Fe（OH）3 colloids. The Fe₃O₄@GA was obtained by calcining the resultant α-Fe₂O₃@GA hybrid at 800 ℃ under nitrogen atmosphere. Compared with pure Fe₂O₃(SBET = 20 m² g^-1, Vp = 0.177 cm3 g^-1), the resultant composites possessed much higher surface area and larger pore volume. The electrochemical properties showed that these iron oxide doped graphene aerogel possessed higher specific capacity, excellent cycle performance and rate performance.（4） 3D a-Fe₂O₃@GA hybrid with high a-Fe₂O₃ dosage was successfully assembled from ferric nitrate with ethylene glycol as solvent. At a current density of100 m A g^-1, Fe₂O₃@GA composites still showed a specific capacity of 745 m Ah g^-1.In brief, the as-prepared composites exhibited more superior electrochemical properties including reversible capacity, cycle stability and rate performance than the corresponding bare oxides. The incorporation of the oxide suppressed the stack of graphene layer. Meanwhile, the abundant porous structure of GA was advantageous to the contact between the electrode surface and the electrolyte, and suppressed the volume expansion effect of the bare oxide during the process of circulation. These results will be helpful for the utilization of GA based materials and the development of LIBs.