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Electrochemical and mechanical processes at surfaces and interfaces of advanced materials for energy storage


Energy storage is a rapidly emerging field. In almost all energy storage applications, surfaces and interfaces are playing dominant roles. Examples are fuel cell electrodes, where electro-catalytic reactions occur, Li-ion battery (LIB) electrodes, where electrolyte decomposition and passivation commence simultaneously, and failure (fracture) of battery electrodes, where surface crack initiation greatly affects battery endurance. The most fundamental chemical, electrochemical, and mechanical problems in energy storage applications originate from surfaces and interfaces. This thesis investigates the electrochemical and mechanical processes at surfaces and interfaces of advanced materials for energy applications. The thesis includes the following five main research topics.

The fuel cell electrodes’ electronic structure was tuned by modifying the metal oxide substrate with oxygen vacancies and/or fluorine doping. The electro-catalyst performance of metal oxide supported platinum nanoparticles was systematically investigated. Compared to Pt nanoparticles on pristine TiO2 support, a two-fold higher activity and three-fold higher stability in methanol oxidation reaction, a 0.12 V negative shift of the CO oxidation peak potential, and a 0.07 V positive shift of the oxygen reaction potential were achieved. Experimental trends were interpreted in the context of an electronic structure model, showing an improvement in electrochemical activity, when the Fermi level of the support material in Pt/TiOx systems was close to the Pt Fermi level and the redox potential of the reaction. The findings of this work provide a better understanding of the substrate effect on electro-catalysis and valuable guidance for selecting the support material of Pt/TiOx systems.

The reference compounds, diethyl 2,5-dioxahexane dicarboxylate (DEDOHC) and polyethylene carbonate (poly-EC), were synthesized and their chemical structures were characterized by FTIR spectroscopy and nuclear magnetic resonance (NMR). The effect of Li-ion solvation on the FTIR spectra was studied by introducing the synthesized reference compounds into the electrolyte. EC decomposition products formed on Sn and Ni electrodes were identified as DEDOHC and poly-EC by matching the features of the surface species forming on electrode surfaces with reference spectra. The results demonstrate the importance of accounting for the solvation effect in FTIR analysis of the decomposition products forming on LIB electrodes.

The composition, structure, and formation mechanisms of the solid electrolyte interface (SEI) on the surfaces of LIB anode electrodes during charge/discharge cycles were investigated with advanced in-situ vibrational spectroscopy. This technique allows us to determine how the SEI properties contribute to the failure of anodes in LIBs used in vehicular applications. Two model surfaces (Au and Sn) were investigated by in-situ attenuated total reflection-infrared (ATR-IR) spectroscopy. The variation of the SEI components on the Au and Sn model surfaces is attributed to surface functional group differences and associated reduction mechanisms. For the first time, we have detected the formation of DEDOHC and Li propionate reductive products by in-situ IR spectroscopy. The results from the model system investigation can be applied to other anode materials of LIBs to improve future battery performance.

The SEI components at the interface of single-crystal Si(100) electrode and electrolyte were studied by ATR-FTIR spectroscopy. High-energy-density Si anodes have large irreversible capacity and poor reliability/durability. These failures are due in part to loss of electrolyte by reduction and the formation of unstable SEI during cycling. To elucidate the mechanism of Si electrode failure during battery cycling, a novel in-situ ATR-FTIR electrochemical cell was constructed, which allows the tuning of the depth of probing in vibrational spectra of molecules at the electrode surface.

Electrochemical experiments, microstructure characterization, and finite element analysis were combined to explore the failure mechanism of single-crystal Si electrodes over cycles. It was found that surface cracks initiating perpendicular to the electrode surface propagate in the depth direction and eventually are deflected along the lithiation boundary, causing electrode material to delaminate. Although crack initiation and growth occur during delithiation, they are essentially driven by irreversible plastic deformation accumulating during the lithiation phase. When the tip of the cracks approaches the lithiation boundary, the low fracture toughness of the crystalline Si (c-Si)/amorphous Si (a-Si) interface, i.e., the “weakest” microstructural path, causes crack deflection along the c-Si/a-Si interface. The findings of this study provide a comprehensive understanding of the failure mechanism of silicon electrodes and guidance for electrode material selection and design optimization.

This ultimate objective of this thesis is the examination of surface and interface material problems in energy applications. The findings of this work reveal a generic behavior comprising interactive chemical/electrochemical reactions and mechanical effects.

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