Soft X-ray Emission Spectroscopy of Liquids and Lithium Battery Materials
Sammanfattning: Lithium ion insertion into electrode materials is commonly used in rechargeable battery technology. The insertion implies changes in both the crystal structure and the electronic structure of the electrode material. Side-reactions may occur on the surface of the electrode, which is exposed to the electrolyte and form a solid electrolyte interface (SEI). The understanding of these processes is of great importance for improving battery performance. The chemical and physical properties of water and alcohols are complicated by the presence of strong hydrogen bonding. Various experimental techniques have been used to study geometrical structures and different models have been proposed to view the details of how these liquids are geometrically organized by hydrogen bonding. However, very little is known about the electronic structure of these liquids, mainly due to the lack of suitable experimental tools.This thesis addresses the electronic structure of liquids and lithium battery materials using resonant inelastic X-ray scattering (RIXS) at high brightness synchrotron radiation sources. The electronic structure of battery electrodes has been probed, before and after lithiation, studying the doping of electrons into the host material. The chemical composition of the SEI on cycled graphite electrodes was determined. The local electronic structure of water, methanol and mixtures of the two have been examined using a special liquid cell. Results from the study of liquid water showed a strong influence on the 3a1 molecular orbital and orbital mixing between molecules upon hydrogen bonding. The study of methanol showed the existence of ring and chain formations in the liquid phase and the dominating structures are formed of 6 and 8 molecules. Upon mixing of the two liquids, a segregation at the molecular level was found and the formation of new structures, which could explain the unexpected low increase of the entropy.
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