Study On Synthesis And Electrochemical Properties Of Iron Oxide Composites As Anode Materials For Li/Na-ion Batteries

Driven by plentiful raw materials, environmental benignity, low processing cost, high theoretical capacities, etc., iron oxides have been regarded as very appealing and potential anode candidates for lithium ion baterries (LIBs) and sodium ion batteries (SIBs). However, its poor cycling stability, low rate capability, and large irreversible capacity, caused by the large specific volume changes during the repeated charge-discharge processes, and the kinetic limitations of its low conductivity intrinsic nature, still strongly hinder its large-scale practical applications. Accordingly, the purpose of this thesis is to enhance the electrical conductivity and structural stability of iron oxide electrodes through combining carbon materials, fabricating various nanostructures and constructing hybrid composites so as to achieve highly desirable electrochemical properties.Both the carbon and MnO2 coated Fe3O4 core-double-shell composite (Fe3O4@(C-MnO2)) with a cube-like morphology has been successfully designed and synthesized by a mild hydrothermal reaction and the layer-by-layer deposition technique. It has been found that the composite exhibits markedly improved lithium storage capability with high reversible capacity, superior rate performance and good cycling stability, which are attributed to the carbon enhancing electrical conductivity, and the the MnO2 nanomaterials being of benefit to increase surface area and accommodate the large volume change of electrodes during battery cycling to some extent. When cycling at 100 mA g-1, this composite delivers a reversible capacity of 979 mAh g-1 after 150 cycles, and even at such a large current density of 2000 mA g-1, it still matains 630 mAh g-1.Porous hollow ?-Fe2O3@TiO2 core-shell nanosphere for use as an anode material in LIBs and SIBs has been successfully fabricated by a simple template-assisted method and surface deposition technique, which has been rarely reported before. The as-prepared a-Fe2O3@TiO2 is composed of a hollow inner cavity and an outer shell with massive mesopores. This porous hollow structure is capable of buffering the large volume variation of ?-Fe2O3 during cycling and preventing the electrode from pulverization and aggregation via the TiO2 layer, as well as providing sufficiently large interstitial space within the crystallographic structure to host alkalis (Li and Na). As a consequence, this hybrid composite exhibits outstanding electrochemical properties, e.g., high specific capacity, excellent cyclability, and satisfactory rate performance for both LIBs and SIBs.A facile, simple, and inexpensive co-precipitation method is used to fabricate diamond-like Fe3O4 nanoparticle/graphene composites for use as LIBs and SIBs electrode materials. In our synthesis, high-temperature treatment, and complicated procedures and apparatus are avoided. Physical characterizations reveal that the as-prepared product is composed of a large fraction of diamond-like Fe3O4 nanoparticles uniformly distributed on/between thin graphene nanosheets. The Fe3O4graphene composite not only reduces the diffusion paths for Li+/Na+ and provides a large electrode/electrolyte interface, but also preserves the structural integrity of the electrode and buffers the severe volume expansion suffered by Fe3O4. Compared to bare Fe3O4, therefore, the as-obtained Fe3O4/graphene exhibits greatly enhanced electrochemical properties for both LIBs and SIBs, including excellent reversible capacity, superior cyclability and good rate performance.Stem-like nano-heterostructural MWCNTs/a-Fe2O3@TiO2 (MCFT) composite, which is composed of TiO2 coated a-Fe2O3 nanoparticles firmly and uniformly anchored on multi-walled carbon nanotubes (MWCNTs), has been successfully fabricated via a facile layer-by-layer deposition technique and further investigated as an anode material for LIBs. It has been found that the as-obtained MCFT composite exhibits greatly improved electrochemical properties with satisfactory cyclability, admirable rate capability and favourable specific capacity for high-performance LIBs in comparison to MWCNTs/a-Fe2O3 (MCF), which originates from the novel MCFT nanostructure not only increasing electrical conductivity and shortening Li+ disffusion distance, but also alleviating volume changes and preventing a-Fe2O3 from aggregation and pulverization, thus sticking out the importance of TiO2. Specifically, it is capable of delivering a reversible capacity of as high as 770 mAh g-1 at 200 mA g-1 after 200 cycles and exhibiting a steady discharge capacity of 670 mAh g-1 even cycled at a large current density of 1000 mA g-1.