Development and Characterization of Nano-Structured LiFePO4 Cathode and Li4Ti5O12 Anode Materials for High-Performance Li-Ion Battery
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Lithium-ion batteries with high power density and long lifetime is becoming the leading energy storage technologies for applications such as electric vehicles and large-scale electricity storage. But the state-of-the-art batteries based on current cathode and anode material can hardly meet the requirements of the large-scale applications due to the limitations on power density and safety characteristics. My research has been dedicated to the development and characterization nano-cathode and nano-anode material for new-generation high power lithium-ion batteries. The selected material candidates for cathode is LiFePO4 and that for anode is Li4Ti5O12. The effective combination of solid-state reaction and hydrothermal method has been used to synthesize both LiFePO4 cathode and Li4Ti5O12 anode because of its low cost and availability of the precursors. The goal for my project is to elucidate the fundamental processes for controllable synthesis of stable LiFePO4 cathode and Li4Ti5O12 anode nanomaterials. The first part of my thesis is the controllable synthesis and performance characterizations of carbon-coated LiFePO4 nanomaterials. A variety of analytical techniques such as x-ray diffraction, scanning and transmission electron microscopy (TEM, HRTEM), electron diffraction, and X-ray photoelectron spectroscopy are applied to investigate LiFePO4 morphologies and phase structures on the nanometer scale. Well-ordered olivine LiFePO4 crystal with a homogenous carbon coating of ~ 3 nm thickness is clearly revealed. The state-of-the-art structural characterization techniques provide a comprehensive view of the correlation between structure and performance of these LiFePO4 cathode nanomaterials. The nanostructures characteristics and the amorphous carbon-coating has been demonstrated to improve the electrical conductivity by reducing the path of both electron transfer and lithium ions diffusion, thereby is beneficial to improve electrochemical performance of these LiFePO4 nanomaterials. The excellent performance in terms of enhanced rate capability, good cycling performance, and high discharge capacity, should enable the development of high power LiFePO4 batteries. More importantly, the practical performance of these carbon-coated LiFePO4 nanomaterials as cathode was performed with a prototype of 18650-type battery cell manufactured by using the commercial graphite as the anode active materials. The remarkable rate capability and cycling performance are clearly demonstrated in the prototype of LiFePO4 battery cell. The second part of my thesis is the facile synthesis and performance evaluation of carbon-coated spinel Li4Ti5O12 nanomaterials. Spinel Li4Ti5O12 has been regarded as an attractive anode material for the development of high-power lithium-ion batteries because of its unique attributes of high safety and rate capability. Carbon-coating has been proved to be an effective method to improve electronic conductivity of Li4Ti5O12 anode materials. It is critically important to investigate in depth the influence of the carbon-coating on the electrochemical performance. Comparative nanostructure analyses and various electrochemical testing demonstrated that these Li4Ti5O12 anode nanomaterials have the improved capacitive, high-rate, and enhanced cycling performance. These improved lithium storage properties can be attributed to the combination of uniform thin carbon-coating and high-purity spinel Li4Ti5O12 nanocrystal, which increases electron transport and facilitates lithium-ion insertion/extraction simultaneously throughout the electrode, making it a highly promising anode material for use in the development of high power density lithium-ion batteries. The practical comparison of the carbon-coated Li4Ti5O12 nanomaterial and the commercial Li4Ti5O12 sample was evaluated in half cells with lithium as the negative electrode. More interestingly, the improved cycling performance is demonstrated in the Li4Ti5O12 battery cell. Finally, the future outlook of the research directions and key developments of spinel Li4Ti5O12 anode and olivine LiFePO4 cathode are proposed from view of scientific project and industrial demand. The practical attempt is to investigate the effective combination of Li4Ti5O12 anode and LiFePO4 cathode to design the leading nano-battery of Li4Ti5O12/LiFePO4 with a high degree of safety, long cycle life and rapid charge for various potential applications. In addition, the prospect of newly development of graphene-Li4Ti5O12 anode and graphene-LiFePO4 cathode hybrid nanocomposite materials for next-generation of green and sustainable lithium-ion batteries is also presented in the last Chapter.