Preparation And Electrochemical Performance Of Iron Oxide And Iron Oxide-graphene Nanocomposites

Lithium ion batteries?LIBs? are widely used as the power source for portable electronic devices and electrical/hybrid vehicles, owing to their high energy density, long life-span, and environmental friendliness. As an important member of transition metal oxides, iron oxides, such as Fe₂O₃ and Fe₃O₄, have recently received increasing attention as very promising anode materials for rechargeable lithium-ion batteries?LIBs? because of their high theoretical capacity, non-toxicity, low cost and safety. However, iron oxides suffer from poor recyclability, which is partly caused by the drastic volume change during the charge/discharge process. The low conductivity of iron oxides also leads to severe loss of capacity. To solve the problems, the main method is as follows: synthesis of the nanostructured materials and preparation of the carbon matrixes hybrids to keep the structural integrity and improve the electrical conductivity of iron oxides anode materials.Based on the above key scientific issues, Fe₂O₃ nanodiscs have been prepared via a facile solvothermal method with high yield. The corresponding porous Fe₂O₃ nanodiscs can be easily obtained by calcining the Fe₂O₃ precursor in air flow. While annealing in the mixture gas of H₂ and Ar?the mol ratio of H₂-to-Ar was 5:95?, Fe₂O₃ nanodiscs can be converted to the porous Fe₃O₄ nanodiscs. Meanwhile, we propose a facile one-step strategy to prepare iron oxide and graphene nanocomposites under hydrothermal conditions, and the electrochemical properities of the composites are investigated. The main contents of this thesis are as follows:?1? The hematite nanodiscs exposing?001? facets have been obtained by a one-pot solvothermal process, using ferric nitrate as the source of iron and urea as precipitating agent, glycerol and water as solvent. The results show that the morphologies and structure of the materials can be influenced by reaction temperature, the content of the urea and water. The oriented growth of hematite perpendicular to the [001] direction has been ascribed to the affinity of glycerol on the?001? facet. However, the results show that urea is not the crucial factor for the formation of the nanodiscs. The glycerol plays the mainly role in the forming process of the Fe₂O₃ nanodiscs, which adsorbed on the [001] crystal orientation, inhibiting the growth on the?001? crystal plane. The growing process of the Fe₂O₃ nanodiscs is ascribed to the precipitation–dissolution–growth mechanism accompanied by oriented growth, in which small particles dissolved and recrystallized onto a preferred growing?001? direction of the large particles. Porous Fe₂O₃ nanodiscs exhibit excellent rate capability and cycling properties as the anode for Li-ion batteries, which can be ascribed to the reducing diffusion distance of Li+ ions and increasing reactivity of the material due to the nanosize effect. In particular, the nanopores can release the volume swell to make the materials more stable in the process of charging and discharging. After the calcination treatment in the mixture gas of H₂ and Ar, Fe₃O₄ nanodiscs is prepared, which can greatly improve the electrochemical performance of the electrode.?2? Based on the process of preparing Fe₂O₃ nanodiscs, Fe₂O₃ nanosheet and reduced graphene oxide composites are produced by an in situ hydrothermal method. The results show that the morphology and structure of Fe₂O₃ nanosheet in the Fe₂O₃ and graphene hybrids are unchanged during the adding of the grapheme, and the Fe₂O₃ nanosheet is uniformly dispersed into graphene. Graphene sheets not only accommodate the volume variation but also improve the electrical conductivity of the overall electrode during the electrochemical process. When the content of RGO is 22.3% in the hybrids, the composite shows the specific capacity of 560 mAhg^-1 at the current density of 5 Ag^-1, and remains at 850 mAhg^-1 after 50 cycles at the current density of 500 mAg^-1.?3? The Fe₃O₄ nanoparticles and graphene hybrids are prepared in a solvent of containing Fe³⁺ and organic acid. By adjusting the content of urea and H₂O₂, pure Fe₃O₄/graphene composite is synthesized. Meanwhile, the size of Fe₃O₄ nanoparticles decreases firstly and then increases with the the amount of H₂O₂ increasing. Because of the cutting effect of free radicals generated by the hydrogen peroxide decompositing, the size of the graphene becomes smaller and the more porous graphene appears in the obtained composites, which provide a more well contact with electrolyte and benefit the fast transfer of Li⁺ ion. As a consequence, the Fe₃O₄/graphene composites show the good specific capacity and the high rate performance.