Identifier

etd-12082010-161626

Degree

Master of Science in Mechanical Engineering (MSME)

Department

Mechanical Engineering

Document Type

Thesis

Abstract

We have investigated the enhanced Li-ion intercalation properties of two different materials, which are V2O5-WO3 composite and surface-coated LiCoO2. A simple and novel solution processing method is employed to prepare V2O5-WO3 composite films that demonstrate enhanced Li-ion intercalation properties for applications in Li-ion batteries or electrochromic displays. This solution processing method employs precursors that only contain the elements of V, W, O and H, which avoids impurity elements such as Na that has been commonly used in other solution methods (e.g. using precursors of sodium metavanadate and sodium tungstate solution). The V2O5-WO3 composite films show enhanced Li-ion intercalation properties compared to pure V2O5 and WO3 films. For example, at a high current density of 1.33 A/g, the V2O5-WO3 film with a V2O5/WO3 molar ratio of 10/1 exhibits the highest discharge capacities of 200 mA•h/g at the first cycle and 132 mA•h/g after 50 cycles, while pure V2O5 film delivers discharge capacities of 108 mA•h/g at the first cycle and 122 mA•h/g after 50 cycles. The enhanced capacity of the composite films is ascribed to the reduced crystallinity, increased porosity and thus the enhanced surface area. Both cyclic voltammogram and chronopotentiometric curves of the V2O5-WO3 film with a molar ratio of 10:1 are distinctively different from those of pure oxide films, suggesting a different Li-ion intercalation process in the V2O5-WO3 film with the molar ratio of 10:1. For surface-coated LiCoO2, a molten salt synthesis method is employed to prepare LiCoO2 powders with submicron size, and ultrathin conformal Al2O3 or ZnO layers are coated on LiCoO2 electrodes by utilizing atomic layer deposition. The Al2O3-coated LiCoO2 electrode exhibits improve cycling stability during the electrochemical measurements, while the ZnO-coated LiCoO2 electrode does not show an improved cycling performance. The Al2O3 coating can prevent the direct contact between LiCoO2 and the electrolyte, and thus suppress the cobalt dissolution and side reaction. Therefore, LiCoO2 with nanosized thin Al2O3 coating exhibits improved cycling performance. In contrast, the ZnO coating is not electrochemically stable, so the ZnO-coated LiCoO2 electrode exhibits a similar cycling performance with the bare LiCoO2 electrode.

Date

2011

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Wang,Ying

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