Identifier

etd-07022012-221936

Degree

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Document Type

Dissertation

Abstract

With the rapid development of electronic devices, electrical vehicles and space aerocrafts, rechargeable lithium-ion batteries (LIBs) have attract numerous interests due to high energy and power density, long lifespan and low self discharge. Spinel LiMn2O4 is a promising cathode material for novel LIBs thanks to high working potential, easy synthesis and low cost, but its severe capacity drop mostly due to inevitable reactions with electrolytes is a drawback. In the dissertation, ultrathin and highly-conformal Al2O3 coatings are grown on the surface of micro-sized and nanosized LiMn2O4 with atomic layer deposition. Thickness of Al2O3 ALD coatings can be precisely controlled by varying ALD growth cycles, notably with a growth rate of 1.5 Å per cycle on nano-LiMn2O4 particles. The coatings improve electrochemical performance of LiMn2O4 cathodes in LIB. For example, micro-LiMn2O4 cathode coated with 10 Al2O3 ALD layers shows higher discharge capacity of 53.7 mA h g-1 than bare micro-LiMn2O4 cathode (40.2 mA h g-1) over 100 cycles in half cells, and nano-LiMn2O4 cathode coated with 6 Al2O3 ALD layers delivers a capacity retention efficiency 23% higher than that of bare nano-LiMn2O4 cathode over 100 cycles in full cells. One reason for popularity of nanostructured TiO2 is its higher working potential and better safety as anode material in high-power LIBs. The effect of morphology and phase structure of anodic TiO2 nanotube arrays on their capacity and cycleability in LIBs is investigated, as well as their thermal and electrochemical stability. The other reason is its broad use in dye-sensitized solar cells (DSSCs). One-dimensional TiO2 nano structures have large surface area for dye loading and ordered paths for fast electron transport, good for enhanced conversion performance of TiO2-based DSSCs. In this project, single-layer, double-layer and bamboo-type TiO2 nanotube arrays are synthesized by anodic oxidation of Ti foils under constant- or alternating-voltage conditions. Their growth mechanisms are carefully explored to achieve morphology control, based on dominant ion diffusion-controlled mechanisms. Various bamboo-type TiO2 nanotubes are produced by tuning the holding time of high-voltage and low-voltage steps, and denser ridges on tube walls are found to favor performance of TiO2 nanotubes in DSSCs.

Date

2012

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Wang, Ying

DOI

10.31390/gradschool_dissertations.3303

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