Date of Award

2016

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Materials Science and Engineering (PhD)

Administrative Home Department

Department of Materials Science and Engineering

Advisor 1

Reza Shahbazian Yassar

Advisor 2

Jun Lu

Committee Member 1

Stephen Hackney

Committee Member 2

Gregory Odegard

Abstract

Alpha (α-) MnO2 is a well know transitional metal oxide possessing one dimensional 2×2 (4.6 × 4.6 Å2) tunnels for accommodation of various ions. Such a characteristic tunneled structure has enabled the wide applications of α-MnO2 in the fields of ion exchange, molecular sieves, biosensor, catalysis and energy storage. This PhD dissertation focuses on the dynamic study of ion transport functionality of α-MnO2 at atomic level using an aberration corrected scanning transmission electron microscopy equipped with a special holder with a scanning tunneling microscopy probe.

The wide application of in situ TEM studying the dynamic behaviors/reactions in rechargeable lithium ion battery is first reviewed. Li+-tunnel interaction during lithiation of a single α-MnO2 nanowire was then systematically studied in situ at sub-Å resolution. An asynchronous tunnel expansion was for the first time captured with an ordered Jahn-Teller distortion theory proposed and confirmed further by DFT. Reversible Na+ insertion in the 2×2 tunnels of α-MnO2 is also explored and the tunneled structure is found to be less stable during sodiation than lithiation, which is explained by the larger Na+ ionic size and thus stronger Na+-tunnel interaction. The effect of large cations (K+) occupying the center of 2×2 tunnels on the electrochemical performance of α-MnO2 as a LIB cathode is systematically studied by controlling K+ concentration. It is found that the presence of K+ improves both the electronic conductivity and Li+ diffusivity of α-MnO2 nanowires, leading to superior discharge rate performance compared to the ones without K+ presence. The last project explores the oriented attachment (OA) growth mechanism of α-MnO2 in aqueous solution. The atomistic formation mechanism of the OA interface is demonstrated based on sub-Å analysis of the edge structures of related planes of α-MnO2. The tunnel-based nature of OA interface is evidenced by direct atomic imaging. The role of surface atomic arrangement at single-tunnel level in directing the self assembly of α-MnO2 nanowires is clearly illustrated with strong DFT theory support.

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Available for download on Thursday, June 01, 2017

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