Electrochemical Performance Of TiO₂ Nanotube Arrays For Lithium Ion Insertion

Lithium ion battery (LIB) has become the most important energy storage device since 1990s, because of its several advantages, such as high operation voltage, small pack volume and weight, high specific capacity, little memory-effect, no environmental pollution, and low self-discharge and long cycle life. Recently, due to the fast development of hybrid vehicles, more and more research attention focuses on LIBs with high energy and high power density. At present, most commercial LIBs are based on graphitic carbon anode materials, which are unsafe and suffer from serious capacity fading under high rate operations. Therefore, a new anode material has to be developed. Titania has advantages of high lithium-ion intercalation voltage, good cycle ability, and high capacity at high current density, and thus turns out to be the novel anode material with safety and high power density.This thesis adopted anodic oxidation method to prepared titania nanotubes arrays. The effect of morphology and crystal structure of the nanotubes on their electrochemical performance was investigated.Firstly, we comprehensively investigated the processing of anodic oxidation parameters with glycerol electrolyte and glycol electrolyte, respectively. The experimental results shows that the length of the nanotubes is about 1.2μm using mixed electrolyte of glycerol and water, while the length reaches 90μm in the case of using glycol electrolyte. Based on SEM and TEM observations, the thickness of the nanotubes are found to decrease from 80 nm to 45 nm and the length increase from 18μm to 90μm as the electrolyte temperature increase from 0℃to 20℃.Then we investigated the effect of Zr doping into the titania nanotubes on their morphology characterization, the phase transition and the electrochemical performance. Zr-doped titania nanotubes arrays were fabricated by anodizing of the Zr-Ti thin plates. Zr-doping can retard the phase transition from anatase to rutile, and increase the array length. The length of the nanotubes prepared with glycerol electrolyte increased by 0.5μm, and the surfaces of the nanotubes became smoother. Cyclic voltammetry revealed that the lithium-ion declaration voltage of Zr-doped array is lower than pure titania array by 0.05V at 1mV/s.Finally, we investigated the effect of the phase composition, morphology on the electrochemical performance. We find that the rate capacity performance of amorphous titania nanotubes is superior to anatase and rutile TiO₂ nanotubes. At current density of 1A/g, the capacity of amorphous nanotubes is 220mAh/g and this of anatase is 150mAh/g. While at current density of 10A/g, the capacity is 175mAh/g for amorphous nanotubes and only 60mAh/g for anatase. Cyclic voltammetry shows the lower lithium-ion insertion voltage in mixed phases of anatase and ruitle than that in pure anatase. The highest specific capacity is obtained with 90μm-length TiO₂ nanotubes mixing with carbon black (180mAh/g) compared to nanoparticles (80mAh/g) and the 90μm-length TiO₂ nanotubes array (60mAh/g). Through comparing the electrochemical performance of different length nanotubes array at a current density of 0.1A/g, we find that the capacity decreases as the array length increases. The reason is ascribed to the higher IR drop due to the worse electric conductivity of long array than short array. The capacity is 110mAh/g for 27μm nanotubes array, and 60mAh/g for 90μm array.