Synthesis And Electrochemical Properties Of Nanostructural Transition Metal Oxides As Anode Materials For Li-ion Batteries

Lithium-ion batteries（LIBs） have been considered as one of the most promising energy storage system because of their high energy density, long lifetime and environmental benignity. Due to good cycling performance, graphite has been widely used in commercial LIBs. However its lower theoretical capacity limits the practical applications of LIBs in high power fields. To meet the demands for high energy density batteries, it is essential to develop new electrodes from high theoretical capacity, low cost and environmentally benign materials.With the advantages of their high irreversible capacity and abundance, transition metal oxides（TMOs） have attracted much attention. However, they are suffered from the problems of low electronic conductivity and large volume change during cycling process, which result in big initial capacity loss and poor cycling performance.In this thesis,synthetic strategies of nanostructral TMOs, carboneous TMOs, and morphology control were applied to improve the electrochemical performance of TMOs as the anodes of LIBs. The detailed contents as followings:（1） Carbon-coated α-Fe₂O₃ nanostructures as the anode of Li-ion battery have been deposited on the stainless steel substrate by a facile pyrolysis of ferrocene. The anode shows a high specific discharge capacity of 1138 mAhg^-1 after 300 cycles at the current density of 500 mAg^-1, and maintains a good capacity of 458.8 mAhg^-1 even cycled at the high current density of 10000 mAg^-1. The high capacity can be associated to the nanostructure and the carbon layer coated on hematite. Moreover, the mechanism for the capacity evolution with cycling has been investigated by scanning transmission X-ray microscopy（STXM）. The results reveal that the detailed composition and electronic structure change in the cycling process. Fe chemical state plays a critical role in the capacity evolution and a low oxidation state of Fe（such as Fe2+） might reduce the capacity by trapping Li+ ions, and the recovery of Fe2+ to hematite(Fe³⁺) significantly enhances the capacity. Data also show the growth and inhomogeneous distribution of solid electrolyte interphase（SEI） layer containing carbon-based film, Li2 O and Li2CO3. The facile synthesis of carbon-coated α-Fe₂O₃ opens an efficient way for large-scale anode production in Li-ion battery and the STXM study provides new insight into the mechanism of hematite-based Li-ion battery.（2） Fe₂O₃ coated TiO₂ nanotubes（Fe₂O₃@TiO₂ nanotube） composites anodes for lithium-ion batteries（LIBs） have been prepared by hydrothermal method and atomic layer deposition（ALD） technique. The composites anodes show a specific discharge capacity of 450 mAhg^-1 after 150 cycles at the current density of 200 mAg^-1, which is approximately two times of pure TiO₂ nanotubes. Even at a high current density of 3200 mAg^-1, the composite anodes still exhibit a good capacity of 198 mAhg^-1, more than three times higher than that of pure TiO₂ nanotubes. The good reversible capacity and rete capability of composite anodes indicate the cumulative effect of Fe₂O₃ on TiO₂ nanotube by the integration of structural stability of TiO₂ and high theoretical capacity of Fe₂O₃.（3） Atomic layer deposition（ALD） technique was employed to deposit maghemite（γ-Fe₂O₃） nanoparticles on carbon nanotubes（CNTs） to synthesize the γ-Fe₂O₃@CNTs composite, which exhibits a superior lithium storage performance. A high discharge capacity of 1148.8 mAhg^-1 was obtained after 300 cycles at the current density of 500 mAg^-1. The good electrochemical performance of the composite anode can be attributed to the conductive network formed by CNTs which can enhanced the conductivity of the electrode and the small size of the nanoparticles which can alleviate the volume change of the electrode.Furthermore, the results of electron energy-loss spectrum（EELS） reveal that the incomplete phase conversion of γ-Fe₂O₃ reduces the capacity and the recoveries of low oxidation state to Fe³⁺ significantly increase the capacity.（4） The porous Co₃O₄ nanostructures were synthesized by a simple template method. From the SEM and TEM images, we can know that the porous Co₃O₄ nanostructures show a porous nanosheet structure which is consisted of nanoparticles. As anode materials for LIBs, the porous Co₃O₄ nanostructures show a good electrochemical performance. A high discharge capacity of 1281 mAhg^-1 was obtained after 400 cycles at the current density of 500 mAg^-1. The good electrochemical performance of the porous Co₃O₄ nanostructures could be ascribed to the uniform porous structure of the anodes which can shorten the diffusion distance of Li+ and electron and could alleviate the volume change of the anode as well.