Preparation And Lithium Storage Performance Of Transition Metal Oxides With Various Morphologies

The anode materials are dominated by carbon materials for commercialized lithium ion batteries. But the relative low theoretical specific capacity and the safety problems make the researchers focus on seeking for new-type anode candidates for graphite. Transition metal oxides (MO) are potential anode materials for lithium ion batteries in terms of high theoretical specific capacity, good cycleability and safety. However, low electronic conductivity and large volume change of MO particles lead to poor electrochemical performance of the MO anodes. Controlling the structures of nanoscale MO is believed to be an effective method to the above-mentioned questions. The dissertation synthesized ZnO, NiO and CuO thin film electrodes by solution immersion and electrochemical impregnation methods and prepared CuO/C composite thin film electrodes by self-assemble method. The synthesis conditions and electrochemical performance were investigated. The relationship between electrode morphologies and electrochemical performance were also discussed.ZnO thin film was synthesized on surface of Cu foil by hydrothermal method and the composite of thin film, morphologies and the electrochemical performance were characterized by XRD, SEM and charge/discharge. The results showed that the ZnO owns the structure of wurtzite and the dandelion-liked morphologies, including micro/nano binary ordered structure. Compared with the ordinary ZnO powder material, the dandelion-liked ZnO structure has the advantages of large contact area between electrolyte and electrode interface, fast charge transfer rate and effectively buffering for volume change. The dandelion-liked ZnO thin film exhibited the initial reversible capacity of 980mAh g-1 and remained 310mAh g-1 after 40 cycles. Coil-liked and granular ZnO thin films were prepared by electrochemical impregnation on surface of Cu foil and foam Ni. The coil-liked ZnO owns the diameter of 5~10μm and delivered the initial reversible capacity of 100mAh g-1. The granular ZnO deposited on surface of foam Ni exhibited the initial reversible capacity of 543.5mAh g-1. The exaggerated dimension of the coil structure and the compact deposited layer are adverse for transfer of lithium ions and electrons, which result in sharp capacity decay. The porous net structure of foam Ni enhances the surface area of the electrode and improves the electron transfer rate, finally enhancing the capacity.NiO thin films with different morphologies on Cu and foam Ni substrates were prepared by sol-gel and electrochemical impregnation methods, respectively. The morphologies and the electrochemical performance were investigated. The results showed that the thin film prepared by sol-gel method was composed of the agglomeration of island-liked NiO. It exhibited an initial discharge capacity of 1502mAh g-1 and the charge capacity of 564.3mAh g-1 with an initial coulombic efficiency of 37%. NiO thin films on Cu foils prepared by electrochemical impregnation own the massive structure and showed an initial discharge capacity of 997mAh g-1 and the initial charge capacity of 698.2mAh g-1. After 50 cycles, the capacity is remained above 651.8mAh g-1. The excellent electrochemical performance is attributed to the unique net structure, which improves the conductivity of NiO, enhances the contact area between electrode and electrolyte interface, and buffers the volume change during cycles.Network-like, flower-liked and spherical-like CuO thin films were prepared by controlling reaction time and the compositions of solution. The results showed that the diffusion paths of Li ions and electrons were shortened and the stability of the electrodes was improved as the particle size of CuO was reduced to nanoscale, and the resulting films exhibited high reversible capacity, long life and high rate capability. Network-liked CuO films were obtained in the solution of 0.25mol L-1 NaOH/9mmol L-1SDS/0.1mol L-1K2S2O8, spherical-like CuO films were prepared in the solution of 2mol L-1NaOH/0.1mol L-1K2S2O8 and flower-liked CuO films were synthesized in the solution of 0.25mol L-1NaOH/35mmol L-1SDS. X-ray diffraction (XRD) measurement revealed that the CuO films with different morphologies were all monoclinic phase. Flower-liked CuO thin films showed better cycleability than that of network-liked and spherical-like CuO films; the network-liked CuO thin film electrode has the relative large initial irreversible capacity but the cycleability is stable; the spherical-like CuO films showed the highest initial coulombic efficiency. The structures of three types of films were optimized in term of electrochemical performance and the results confirmed the dependence of the electrochemical properties on microstructures. It was believed that large surface area, stable structure and good conductivity were favorable for performance improvement.Sisal-liked CuO/C composite thin films were obtained by the heat treatment of self-assembled Cu(COOC6H4NH2)2 clusters on copper substrates. The as-prepared composite films possessed the binary-structure on both micro/nanometer scales, which enlarged the contact area of electrode/electrolyte and stabilized the structure of composite. Carbon phase in the composite improved the conductivity and suppressed the agglomeration of CuO particles. The as-prepared films exhibited a reversible capacity of 600mAh g-1 after 60 cycles.