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Advanced Study of Electrode Materials

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The anode and cathode are main components in a battery system. In this dissertation, Group IV inter-metallics and LiMn2O4 are studied as anode and cathode materials for Li-ion batteries respectively. Preliminary investigation of a multi-valent cathode material for next generation batteries is also introduced. Group IV inter-metallics electrochemically alloy with Li with stoichiometries as high as Li4.4M (M=Si, Ge, Sn or Pb). This provides the second highest known specific capacity (after pure lithium metal) for lithium ion batteries, but the dramatic volume change during cycling greatly limits their use as anodes in Li-ion batteries. We describe an approach to overcome this limitation by constructing electrodes using a Ge/Ti multilayer architecture. In operando X-ray reflectivity and ex situ transmission electron microscopy are used to characterize the hetero-layer structure at various lithium stoichiometries along a lithiation/delithiation cycle. The as-deposited multilayer spontaneously forms a one-dimensional TixGe/Ti/TixGe core-shell planar structure embedded in a Ge matrix. The interfacial TixGe alloy is observed to be electrochemically active and exhibits reversible phase separation (i.e. a conversion reaction). The overall multilayer structure exhibits a 2.3-fold reversible vertical expansion and contraction and is shown to have improved capacity and capacity retention with respect to a Ge film with equivalent active material thickness. The LixMn2O4 (LMO) spinel is a well-established lithium ion battery cathode having a 3D framework with alternating layers of a close-packed array of oxygen, fixed Mn sites and variable Li occupation. We choose LMO epitaxial thin films as our experimental subject based on their advantage of well-defined structure and orientation. Through a systematic pathway, we optimized the deposition condition and conducting buffer layer selection for LMO epitaxial thin film preparing. In the following operando experiments on LMO epitaxial thin films, we observed the reorganization of Mn locations under compressive strain. This artificial strain is achieved through epitaxially bounding LiMn2O4 thin films on SrTiO3 (111) substrates. Multi-valent batteries are promising successors of current Li-ion batteries due to higher energy capacity and density. We report different electrochemical activities (i.e., extraction / insertion) of Mg2+ in epitaxially stabilized MgMn2O4 (MMO) thin films with distinct tetragonal and cubic phases. Tetragonal MMO shows negligible activity, while the cubic MMO exhibits reversible Mg2+ activity with associated changes in film structure and Mn oxidation state. These results demonstrate a novel strategy for identifying the factors that control Multi-valent cation mobility in next generation battery materials.

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  • 01/25/2019
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