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Effect of Nano-Size on Lithium-Ion Batteries : High Voltage LiNi₀.₅Mn₁.5O₄ Spinel and Rechargeable NiO-CuF₂ conversion materials


The worldwide energy crisis boosted the development of environmentally benign energy infrastructures where energy storage is one of the key components. Among various energy storage systems, lithium-ion batteries have been attracted a keen interest as the best candidate for maximizing energy efficiency, however current lithium-ion battery technology is still inadequate to satisfy the requirements of advanced applications such as electric vehicles. LiNi₀.₅Mn₁.5O₄ spinel materials have been considered as a promising positive electrode due to its high operating voltage ̃ 4.7 V (vs. Li/Li⁺) and relatively higher energy density (theoretical specific capacity, 146.72 mA h g⁻¹). In this thesis, elementary polarizations of LiNi₀.₅Mn₁.5O₄ spinel materials with disordered (space group, F d -3 m) and ordered (space group, P 4₃ 3 2) structures are quantitatively analyzed to clarify how the differences in crystallographic structure affect the rate performance. In order to increase the active surface area and reduce the diffusion length, the nanowire electrode was prepared via sol-gel based template synthesis. It is proved that nanostructure can improve instant discharging rate with a reduced charge-transfer and diffusion resistances. As a result of maximized surface area, however manganese ions dissolution results in capacity fading over prolonged cycling. Atomic layer deposition on the surface proved an effective surface protection method. Using a combination of X-ray absorption spectroscopy (XAS) coupled with aberration-corrected scanning transmission electron microscopy (STEM), it is found that the atomic structural transformation at the surface is the main source of the Mn dissolution problem on the conventional spinel LiNi₀.₅Mn₁.5O₄ materials. A new synthesis method, Polyol process, is also introduced which does not suffer from the detrimental dissolution problem. As an alternative to the conventional intercalation materials, CuF₂ conversion materials are also investigated. NiO coated CuF₂ can show the promising rechargeable behavior and its mechanism was disclosed with the help of pair distribution function analysis

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