Preparation And Electrochemical Performance Of One Dimensional Nanomaterials For Lithium Ion Batteries

The protable batteries with higher energy density,better cycliability and reliability are most urged recently by the rapid development of the electronic and communication devices,such as notebook,mobile telephone,camera,aerial and space equipments,as well as Hybrid-Electric Vehicel(HEV).More and more researchers focus on the discoveries of new material concept and the synthesis diversification to meet the increasingly demand for energy conversion and storage.Scientists have realized that breakthrough in the electrode materials is the key point for the next generation of the lithium ion batteries.Nano-materials and nano-composites are expectable for the higher capacity and better cycliability due to their special microstructure and micro morphology,which have higher specific surface area and bigger pore spaces.They can exhibit little polarization,better reversibility and cyliability when charged and discharged under large current density due to the short length of lithium ion diffusion.In this dissertaion,several one-dimensional nano-materials were synthesized by using the methods of combining the wet chemistry or electro-deposition with anodic aluminum oxide(AAO) template.These one-dimensional nano-materials include carbon nano-tubes(CNTs),CNT-coated single-crystal and polycrystalline SnO₂ nanoarrays,CNT-coated polycrystalline LiFePO₄ nanoarrays,tin nanotubes and polycrystalline TiO₂ nanotubes.The wet chemistry methods primarily include the citric acid solution method and the sol-gel method which uses the citric acid as chelating agent.The pH value,water volume and temperature of the sol could affect the chelation degree and then enhance the loading rate of the AAO template.These one-dimensional nano-materials were physically characterized and electrochemically tested as electrode materials for lithium ion batteries.At first(3rd chapter),nanometer-scale alumina template with highly ordered and closely packed hexagonal pore structure was prepared by two-step anodizing process in oxalic acid solution.The nanometer-scale pore holes have high aspect ratio with a pore density of 10¹⁰/cm²,and the pore size can be adjusted by tunning the temperature of the electrolytic cell and the following immersion in phosphoric acid.The electrochemical tests show that AAO template is an electrochemically inactive substrate,which has the least influence on the electrochemical capacities of the nanoarray materials.As a result,AAO template can not only be removed in the follow-up process but also be an inactive substrate to supply the absolute microtubes for the experiments of the microelectrode arrays.CNT has electronic and thermal conductivity in the axis direction like metal,and the large space in and between the nanotubes supplies the potential application in the fields of hydrogen storage,lithium ion storage and super capacitor.In the fourth chapter,large scale and uniform CNTs and CNT/Al₂O₃ composite were successfully synthesized by using citric acid solution-AAO template method.The wall thickness of the nanotube and the aspect ratio can be adjusted.These CNT arrays have potential application in Field Emission Display(FED) devices.Further investigation shows that the nanopores of AAO have catalyze the graphization of the CNTs during 450℃-600℃,while the interfaces prefer to catalyze the crystallization of the alumina substrate for the tetrahedral Al₂O₃ above 600℃,which is lower than the thermal crystallization temperature of the AAO.The electrochemical tests show that CNT/Al₂O₃ composite exhibits higher lithium storage capacity compared with CNT material,with excellent cycling property and relatively high rate capacities.However, the electronic conductivity of the composite should be improved.In the fifth chapter,the lithiation and delithiation properties of two kinds of SnO₂ powders were investigated at first.Then sol gel-AAO template method was applied to synthesize CNT-coated single-crystal and polycrystalline SnO₂ nanoarrays.This synthesis process is instructive for preparation of the other CNT-coated single-crystal or polycrystalline oxides nano-arrays.According to the results,the crystal SnO₂ exhibits higher capacity and higher coulombic efficiency in the 1st cycle than the amorphous SnO₂ due to the less charge transferring impedance and the faster lithium ion diffusion.In the other hand,the amorphous SnO₂ exhibits better cycliability than crystal SnO₂ because the amorphous structure conduces to forming the highly dispersed tin atoms during the first reduction process.The tin atoms aggregation is avoided,and then good cycliability is obtained.According to the one-dimensional SnO₂ materials,the loading rate of the single-crystal SnO₂ nanoarrays is high,with a length of 1μm or so.Further investigations show that the single-crystal nanowires have random crystallization directions.As a result,the XRD pattern of the nanoarrays shows no strongly preferential orientation.Furthermore,the CNT-coated polycrystalline SnO₂ nanoarrays were successfully synthesized by decreasing the pH value and increasing the water volume of the sole precursor,of which the nanowire length is about several micrometers.Electrochemical tests show that CNT-coated SnO₂ nanoarrays obtained relatively high capacities and greatly improved cycliability due to the combined superiority of both the fast lithium ion diffusion and the good axial conductivity.It is thought that the limited free space in the nanoarrays and the dimensional confinement of the nanowires helped a lot to avoid large volume expansion when lithiation and delithiation processes performed,which promoted the cycling performance.In the sixth chapter,several tin films were electrodeposited with different current densities using constant current technique.The morphology,structure and the electrochemical properties of these tin films were investigated.Then we used the AAO template and constant voltage technique to electrodeposit the one-dimensional tin material.Thermaldynamically steady tin films were obtained at low depositing current,which have compacter electrode structure and firmly packed crystalline interface.The tin film and copper substrate also conected to each other firmly.As a result,these tin films exhibited the excellent cycliability while the capacity is low.It is considered that the compact electrode structure and the firmly packed crystalline interface help a lot to the integrity of the electrode during lithiation and delithiation processes,which greatly improved the cycliability,although the capacity was limited by the low lithiation and delithiation processes due to the compact electrode structure. Inversely,the higher capacities and worse cycliability are got for the tin films electrodeposited at high current densities,due to the less compact electrode structures and crystalline structures.According to the one-dimensional tin deposition,it is tubelike shape.The formation of the tin nanotubes is related to the porous structure of the AAO template and the sputtered porous Au film as well.Besides,the inclination of the adsorption of the citric acid chelating agent to the alumina pore wall is accountable.Compared to the tin films,tin nanotube array exhibits further improved cyliability.For example,tin film deposited at 0.5mA/cm² exhibited the delithiation capacities of 497 mAh/g and 88 mAh/g in the 1st and 20th cycle,while tin nanotube arrays exhibited the delithiation capacities of 423 mAh/g and 223 mAh/g in the 1st and 20th cycle,which has the better capacity retention ability.As a result,the nanotube morphology improved the electrochemical cyliability due to the dimensional confinement of nanosize effect to a certain degree although the essential structural changes during the alloying process are still unresolved.The cycliability of the tin nanotube arrays still needs further improvement.In the seventh chapter,ultrasonic and long-time immersion measurement were used to initiate the primary synthesis of TiO₂ nanotubes through sol-gel-AAO route,then the physical chemistry and the electrochemical properties of the TiO₂ nanotubes were characterized.The isoelectric points of TiO₂(5～6) and Al₂O₃(6～8) are close to each other.That means the surfaces of these two oxides are both positively charged in the high acidic solution,and they will be repulsed by each other.Accordingly,the loading of the TiO₂ into the AAO pore holes will be relatively difficult.Ultrasonic and long-time immersion measurements are effective methods to load TiO₂ sol into the AAO pore holes although the loading rate is low.The end products are polycrystalline TiO₂ nanotubes with anatase phase,which exhibited the promoted electrochemical cycling property and the promoted lithiation dynamics campared to the TiO₂ powder.In the last,we primarily synthesized CNT-coated polycrystalline LiFePO₄ nanowire arrays,which further proved the validity of the sol-gel-AAO method for the preparation of CNT-coated one-dimensional semiconductor materials.