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Abstract: With emerging interest in sodium‐ion battery (SIB) technology due to its cost effectiveness and high earth abundance of sodium, the search for an optimised SIB system becomes more and more crucial. It is well known that the formation of a stable solid electrolyte interphase (SEI) at the anode in Na‐based systems is a challenge due to the higher solubility of the SEI components compared to in the lithium‐ion battery case. Our knowledge about the formation and dissolution of the SEI in SIBs is so far based on limited experimental data. The aim of the present research is therefore to enhance the understanding of the behaviour of the SEI in SIBs and shed more light on the influence of the electrolyte chemistry on the dissolution of the SEI. By conducting electrochemical tests including extended open circuit pauses, we study the SEI dissolution in different time domains. With this, it is possible to determine the extent of self‐discharge due to SEI dissolution during a specific time. Instead of using a conventional separator, β‐alumina is employed as a Na‐conductive membrane to avoid crosstalk between the working electrode and Na‐metal counter electrode. The SEI composition is monitored using synchrotron‐based soft X‐ray photoelectron spectroscopy (SOXPES). The electrochemical and XPS results show that the SEIs formed in the studied electrolyte systems have different stabilities. The relative capacity loss after a 50‐hour pause in the tested electrolyte systems can be up to 30%. Among three carbonate‐based solvent systems, NaPF 6 in ethylene carbonate: diethyl carbonate exhibits the most stable SEI. The addition of electrolyte additives improves the SEI stability in PC. The electrolyte is saturated with typical SEI species, NaF and Na 2 CO 3 , in order to oppose the dissolution of the SEI. Furthermore, the solubilities of the latter additives in the different solvent systems are determined by inductively coupled plasma – optical emission spectrometry.

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Abstract: A major limit of electric vehicle performance is the energy density of available mobile energy storage systems. Lithium ion (Li-ion) batteries are now well-known and have enabled many portable electronics as well as electronic vehicles. However, for greater market penetration and range competitiveness, a battery with greater practical energy density must be designed. This dissertation focuses on three methods of improving energy density in lithium based batteries: lithium oxygen (Li-O2) batteries, Li-rich cathodes for Li-ion batteries, and high voltage operation of simple transition metal oxide cathodes for Li-ion batteries. To evaluate each of these technologies, the reactivity of each system is monitored. Li-O2 batteries, a “beyond Li-ion" battery technology, have a very high theoretical energy density that is difficult to realize. After initial excitement surrounding the novel chemistry, many challenges associated with Li-O2 batteries have been highlighted in the past decade. Among these challenges, the reactivity of oxygen in the system is one of the most pressing. In this dissertation, the possible stability of LiO2, an advantageous discharge product to the typical Li2O2, is examined alongside binder degradation in the system. As the workings of a Li-ion battery require the removal and intercalation of Li, the theoretical capacity is determined by the amount of Li available for extraction from the cathode. Consequently, a method of increasing energy density in Li-ion batteries is to increase the stoichiometric ratio of Li in the cathode material. These "Li-rich" cathode materials demonstrate large capacities, but must compensate the additional Li with cation double redox or anion redox. The electrochemical cells must be operated to > 4.5 V vs Li/Li+ to reach these additional capacities, and this operation results in greater reactivity and instabilities. This dissertation examines the trends of high voltage instability, especially as it relates to oxygen redox, in these Li-rich cathode materials. Typical Li-ion batteries utilize only a fraction of the theoretical capacity available, only extracting around half of the available lithium from the layered transition metal oxide cathode material during charge. This is due to enhanced degradation mechanisms and reduced cyclability when additional Li is extracted at the necessitated higher voltages. Enabling operation of layered transition metal oxides at high voltages would result in increased capacity without the need for “beyond Li-ion" technologies. But first, the instabilities associated with Li extraction at voltages beyond the typical cut-off must be well-studied. Currently, the stability and degradation mechanisms of cathode materials even as common as LiCoO2 remain unclear. In studies presented in this dissertation, high voltage reactivity for layered transition metal oxide cathode materials is investigated. The main conclusions of the studies presented here are drawn from measurements monitoring the reactivity of each of the aforementioned technologies. Among the measurements presented in these studies, outgassing and gas evolution measurements by differential electrochemical mass spectrometry have proved paramount in utility. As cell reactivity and instability at high voltages is often accompanied by outgassing, these measurements have assisted in the elucidation of instability origins. Practical application of Li-O2 batteries remains elusive as additional instabilities are discovered, Li-rich cathode materials show promise as various methods of mitigating high voltage instabilities are discussed, and the major sources of high voltage reactivity of layered transition metal oxide cathode materials are evaluated here.

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Abstract: The on-surface synthesis of covalently bonded materials differs from solution-phase synthesis in several respects. The transition from a three-dimensional reaction volume to quasi-two-dimensional confinement, as is the case for on-surface synthesis, has the potential to facilitate alternative reaction pathways to those available in solution. Ullmann-type reactions, where the surface plays a role in the coupling of aryl-halide functionalised species, has been shown to facilitate extended one- and two-dimensional structures. Here we employ a combination of scanning tunnelling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and X-ray standing wave (XSW) analysis to perform a chemical and structural characterisation of the Ullmann-type coupling of two iodine functionalised species on a Ag(111) surface held under ultra-high vacuum (UHV) conditions. Our results allow characterisation of molecular conformations and adsorption geometries within an on-surface reaction and provide insight into the incorporation of metal adatoms within the intermediate structures of the reaction.

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Abstract: The search for new wide band gap materials is intensifying to satisfy the need for more advanced and energy effcient power electronic devices. Ga2O3 has emerged as an alternative to SiC and GaN, sparking a renewed interest in its fundamental properties beyond the main β-phase. Here, three polymorphs of Ga2O3, α, β, and ε, are investigated using X-ray diffraction, X-ray photoelectron and absorption spectroscopy, and ab initio theoretical approaches to gain insights into their structure - electronic structure relationships. Valence and conduction electronic structure as well as semi-core and core states are probed, providing a complete picture of the influence of local coordination environments on the electronic structure. State-of-the-art electronic structure theory, including all-electron density functional theory and many-body perturbation theory, provide detailed understanding of the spectroscopic results. The calculated spectra provide very accurate descriptions of all experimental spectra and additionally illuminate the origin of observed spectral features. This work provides a strong basis for the exploration of the Ga2O3 polymorphs as materials at the heart of future electronic device generations.

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Abstract: CuBi2O4 exhibits significant potential for the photoelectrochemical (PEC) conversion of solar energy into chemical fuels, owing to its extended visible-light absorption and positive flat band potential vs the reversible hydrogen electrode. A detailed understanding of the fundamental electronic structure and its correlation with PEC activity is of significant importance to address limiting factors, such as poor charge carrier mobility and stability under PEC conditions. In this study, the electronic structure of CuBi2O4 has been studied by a combination of hard X-ray photoemission spectroscopy, resonant photoemission spectroscopy, and X-ray absorption spectroscopy (XAS) and compared with density functional theory (DFT) calculations. The photoemission study indicates that there is a strong Bi 6s–O 2p hybrid electronic state at 2.3 eV below the Fermi level, whereas the valence band maximum (VBM) has a predominant Cu 3d–O 2p hybrid character. XAS at the O K-edge supported by DFT calculations provides a good description of the conduction band, indicating that the conduction band minimum is composed of unoccupied Cu 3d–O 2p states. The combined experimental and theoretical results suggest that the low charge carrier mobility for CuBi2O4 derives from an intrinsic charge localization at the VBM. Also, the low-energy visible-light absorption in CuBi2O4 may result from a direct but forbidden Cu d–d electronic transition, leading to a low absorption coefficient. Additionally, the ionization potential of CuBi2O4 is higher than that of the related binary oxide CuO or that of NiO, which is commonly used as a hole transport/extraction layer in photoelectrodes. This work provides a solid electronic basis for topical materials science approaches to increase the charge transport and improve the photoelectrochemical properties of CuBi2O4-based photoelectrodes.