Date of Award

2018

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering-Engineering Mechanics (PhD)

Administrative Home Department

Department of Mechanical Engineering-Engineering Mechanics

Advisor 1

Reza Shahbazian-Yassar

Committee Member 1

Craig Friedrich

Committee Member 2

Gregory Odegard

Committee Member 3

Stephen Hackney

Abstract

Cathode crystals in lithium-ion batteries act as the host for the (de)intercalation for lithium ions. The diffusion of lithium ions in layered or tunneled cathode crystals is highly selective along certain crystal plans or directions. Exposed facets of the cathode crystals can greatly affect the diffusion of lithium ions within the electrode, which in return affect the electrochemical performances of the batteries. In this dissertation, layered LiCoO2 and tunnel-based beta MnO2 were selected as two individual systems to evaluate the effect of mechanical stress and exposed crystal facets on the lithium ion diffusion in these two cathode materials, respectively. For the layered LiCoO2 cathode, the effect of mechanical stress on lithium ion diffusion in layered LiCoO2 cathode was investigated using conductive atomic force microscopy (C-AFM). Higher localized mechanical stress could induce more active lithium ion redistribution along the grain boundaries than the grain interiors. The external stress field within 100 nN could induce the resistive-switching effect of the LiCoO2 cathode.

For the tunnel-based beta MnO2 cathode, high-resolution transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) were used to real the lateral facets evolution mechanism in both nanowire and microrod morphologies. The evolution of lateral facets was found to follow the shift from {100} facets to {110} facets because of the relatively high surface energies of the {100} facets compared with {110} facets. Further studies show the micro-sized beta MnO2 was formed through oriented attachment and subsequent direct phase transition from a-KxMnO2 nanowires. The 9 concentration of potassium cations (K+) could be used to control the morphology of the obtained beta MnO2 crystals. The morphology changed from bipyramid prism to octahedron when the concentration of K+ increase from 0.02 M to 0.09 M. The role of K+ cations was revealed to affect both the formation and phase transition of a-KxMnO2 intermediate. The two morphologies were identified with highly exposed {100} and {111} facets, respectively. The effect of crystal facets on the electrochemical and catalytic performance of beta MnO2 was further studies based on the application of these two morphologies in lithium-ion batteries, supercapacitors, and lithium-air batteries systems, respectively. The results show, the highly exposed {111} facets offered beta MnO2 higher lithium ion mobility inside the structure and thus better rate performance because of highly exposed open tunnels. The {100} facets of beta MnO2 offered higher specific capacitance as the electrode for supercapacitors, which is due to the highly exposed Mn centers on the {100} facets compared with {111}. As the cathode catalyst for lithium-air batteries, both facets showed effective catalytic activities in reducing the charge and discharge overpotential; the {111} facets of beta MnO2 was, for the first time, revealed to catalyze a solution-based mechanism for the formation of LiO2 intermediate even in a low donor number electrolyte.

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