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Probing Phase Transformations in Lithium Ion Cathode Materials Using X-ray Imaging
thesisposted on 01.02.2019 by Brian M May
In order to distinguish essays and pre-prints from academic theses, we have a separate category. These are often much longer text based documents than a paper.
This thesis explores the development and application of tools in X-ray imaging and mapping to probe the chemical processes that occur within cathode materials for Li-ion batteries and the insight achieved. Li-ion batteries are governed by redox reactions for which the mechanisms are non-trivial to describe. Achieving a thorough understanding of the function of these materials through oxidation and reduction can reveal intrinsic properties that may ultimately be used to improve battery operation. In this thesis I present synthetic methods employed to prepare samples that conform to the requirements of each analysis tool utilized. Using a hydrothermal setup and high temperature annealing, LiNi0.80Co0.15 Al0.05 O2, a cathode material of significant interest, was synthesized paying special attention to particle size, morphology, and crystallographic orientation, while maintaining electrochemical properties that mimic those of commercial material. Using X-rays for diffraction mapping of individual particles is also presented. Performing diffraction mapping on the nanoscale revealed a behavior in the chemical phase transformation within LiFePO4 primary particles that had never been observed before experimentally. It also provided microstructural maps of the particle, from which chemistry and mechanics were correlated. Finally, operando X-ray diffraction mapping revealed that phase transformations do not occur concurrently for individual secondary particles of LiNi0.80Co0.15 Al0.05 O2and that the rate of reaction within particles changed during the charge-discharge cycle. There is not a known precedent for this behavior, and highlights the necessity for studying phase transformations with high spatial, chemical, and temporal resolution.