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Browsing by Subject "X-ray absorption spectroscopy"

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  • Mäkelä, Ville (2022)
    With their ability to convert chemical energy to electrical energy through electrochemical reactions, rechargeable batteries are widely used to store energy in various applications such as electronic mobile equipment, aerospace aviation, road transportation, power grid, and national defense industry Numerous battery types are available commercially. Lithium ion-based batteries stand out due to several key advantages such as high operating voltage, high specific energy, and long cycle life. They also have a market dominance in a wide range of electric vehicles. However, like all battery technologies, lithium ion-based ones suffer from the effects of aging-induced degradation which can lead to reduced capacity, lifetime, and in some cases even safety hazards. One method of preventing/slowing down these aging reactions is to modify the standard battery materials by using dopants and additives. They are specific impurities purposely introduced into the battery during the manufacturing process. In this master’s thesis, the effect of additives (Mg/Al) on the aging of Li-ion cells was examined by using X-ray absorption spectroscopy, more specifically x-ray absorption near edge structure (XANES). For the experiment, 7 different cells, all containing lithium cobalt oxide as the major component (with 4 having a stoichiometric ration of Li/Co, and 3 being Li-rich), with 5 of them containing Mg/Al as dopants, and 2 containing no dopants were examined using XANES as a function of aging in terms of charge/discharge cycles. The dopants were introduced at different stages of the material preparation, either at the lithiation step or at the synthesis of the precursor. This thesis focuses on the XANES experiment and the data analysis, with extensive literature review on the topic of using additives and dopants. The cells were prepared by the Aalto University. The results showed that of the cells with dopant materials, the cells doped during lithiation stage aged slightly better after cycling than the undoped ones, whereas the cells doped during precursor stage aged worse than the undoped cells. This would suggest that doping might be more effective when done during the lithiation stage.
  • Kivelä, Feliks (2022)
    The crystal structure of magnetite (Fe3O4) involves Fe2+ ions in sites with octahedral (Oh) symmetry and Fe2+ and Fe3+ ions in sites with tetrahedral (Td) symmetry. Magnetite exhibits several interesting physical phenomena, such as the Verwey transition, in which the roles of the different Fe sites are an active subject of research. In the X-ray standing wave (XSW) technique, incoming and diffracted X-ray beams interfere inside a crystal, creating a standing wave with the periodicity of the diffracting atomic lattice. The phase of the wave, i.e. whether the nodes are located on the lattice planes or between them, can be adjusted by finely tuning the diffraction angle. Changing the phase in this way makes it possible to selectively vary the contributions of different atoms and absorption types (dipole versus quadrupole) to the measured total absorption spectrum. Iron K-edge absorption spectra in magnetite were studied in the presence of an XSW in an experiment conducted at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. This thesis presents an analysis of the data gathered during the experiment, with the goal of decomposing the experimentally measured pre-edge peak into its constituent components. The methods used in the analysis include principal component analysis and fitting predicted absorption peaks calculated with the Quanty software to the experimental data. The results show the dipole and quadrupole contributions of the tetrahedral sites responding to changes in the phase of the XSW in opposite ways in a manner consistent with theoretical predictions.