| 1. |
- Ciosek Högström, Katarzyna
(författare)
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The Complex Nature of the Electrode/Electrolyte Interfaces in Li-ion Batteries : Towards Understanding the Role of Electrolytes and Additives Using Photoelectron Spectroscopy
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The stability of electrode/electrolyte interfaces in Li-ion batteries is crucial to the performance, lifetime and safety of the entire battery system. In this work, interface processes have been studied in LiFePO4/graphite Li-ion battery cells. The first part has focused on improving photoelectron spectroscopy (PES) methodology for making post-mortem battery analyses. Exposure of cycled electrodes to air was shown to influence the surface chemistry of the graphite. A combination of synchrotron and in-house PES has facilitated non-destructive interface depth profiling from the outermost surfaces into the electrode bulk. A better understanding of the chemistry taking place at the anode and cathode interfaces has been achieved. The solid electrolyte interphase (SEI) on a graphite anode was found to be thicker and more inhomogeneous than films formed on cathodes. Dynamic changes in the SEI on cycling and accumulation of lithium close to the carbon surface have been observed. Two electrolyte additives have also been studied: a film-forming additive propargyl methanesulfonate (PMS) and a flame retardant triphenyl phosphate (TPP). A detailed study was made at ambient and elevated temperature (21 and 60 °C) of interface aging for anodes and cathodes cycled with and without the PMS additive. PMS improved cell capacity retention at both temperatures. Higher SEI stability, relatively constant thickness and lower loss of cyclable lithium are suggested as the main reasons for better cell performance. PMS was also shown to influence the chemical composition on the cathode surface.The TPP flame retardant was shown to be unsuitable for high power applications. Low TPP concentrations had only a minor impact on electrolyte flammability, while larger amounts led to a significant increase in cell polarization. TPP was also shown to influence the interface chemistry at both electrodes.Although the additives studied here may not be the final solution for improved lifetime and safety of commercial batteries, increased understanding has been achieved of the degradation mechanisms in Li-ion cells. A better understanding of interface processes is of vital importance for the future development of safer and more reliable Li-ion batteries.
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| 2. |
- Mattsson, Andreas, 1976-
(författare)
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Formic acid adsorption and photodecomposition on rutile TiO2 (110) : An in situ infrared reflection absorption spectroscopy study
- 2014
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- TiO2 based photocatalysis is an emerging green nanotechnology that can be used forremoval of pollutants from water and air. It has had an increased research interest, bothfrom an application and fundamental point of view, during the last decades. Despite thismany elementary processes that occur on the photocatalyst surface are not fullyunderstood and are thus limiting our ability to purposefully manufacture more efficientphotocatalytic materials.In this licentiate thesis, the adsorption geometry and photodecomposition of formicacid on differently prepared rutile TiO2 (110) surfaces were investigated. The surface wasprepared by repeated cycles of Argon ion sputtering and annealing. By modifying thisprocedure either reduced, stoichiometric or oxylated surfaces have been obtained. Thesedifferent surfaces are all well-ordered as evident from the obtained low energy electrondiffraction pattern. In addition, a totally disordered surface was also prepared by Argonsputtering alone. Grazing incidence infrared reflection-absorption spectroscopy (IRRAS)employing polarized light with different azimuthal orientations of the TiO2 single crystalwas used to investigate the binding geometry of formic acid (HCOOH) on the surface.Upon adsorption of formic acid on the TiO2 surface, the molecule is deprotonatedresulting in a formate (HCOO-) and a hydrogen (H+) molecule on the surface. The formatemolecules are mainly bridge-bonded to the Ti5c surface atoms with their molecular axisalong the [001] direction. A minority of the formate species was found to adsorb throughhydroxylated oxygen vacancies (or protonated oxygen atoms) and therefore have differentorientations on the surface. For the disordered surface, it was found that the orientation ofthe formate adsorbates are more or less random since no changes in the IRRAS spectraare seen for the different directions of the single crystal. The adsorption geometry for thedisordered surface is also changed, as seen in the shift of the peak positions in the IRRASspectra. This changed adsorption geometry is attributed to exposures of Ti3+ atoms on thesurface, and is a result of the Ar ion sputtering.Irradiation of the HCOO/TiO2 systems by UV light (365 nm, 2 mW/cm2) showed onlya small change in formate coverage after 100 minutes of illumination. The decrease waslargest on the disordered surface and miniscule on the ordered surface. These results werecompared with those obtained on rutile nanoparticles at ambient conditions. Thecomparison shows that the adsorption geometry for the nanoparticles is similar to that ofthe ordered single crystal surfaces and that the photodecomposition rate is about a factorof 30 higher on the nanoparticles than on the disordered surface. This difference isexpected as the single crystal experiments were performed in vacuum, where the supplyof O2 electron acceptors and OH/H2O donors from the gas phase is limited.These results shows that the rutile TiO2 (110) surface is a good model system forfundamental studies of nanoparticle systems and that the presence of hydroxylated oxygenvacancies and protonated oxygen atoms are important for the reactivity of the TiO2surface and must be included in the description of surface reactions on rutile surfaces.
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| 3. |
- Plogmaker, Stefan
(författare)
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Techniques and Application of Electron Spectroscopy Based on Novel X-ray Sources
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The curiosity of researchers to find novel characteristics and properties of matter constantly pushes for the development of instrumentation based on X-radiation. I present in this thesis techniques for electron spectroscopy based on developments of X-ray sources both in time structure and energy. One part describes a laser driven High-Harmonic Generation source and the application of an off-plane grating monochromator with additional beamlines and spectrometers. In initial experiments, the source is capable of producing harmonics between the 13th and 23rd of the fundamental laser 800 nm wavelength. The intensity in the 19th harmonic, after monochromatization, was measured to be above 1.2·1010 photons/second with a repetition rate of 5 kHz. The development of a chopper system synchronized to the bunch clock of an electron storage ring is also presented. The system can be used to adjust the repetition rate of a synchrotron radiation beam to values between 10 and 120 kHz, or for the modulation of continuous sources. The application of the system to both time of flight spectroscopy and laser pump X-ray probe spectroscopy is shown. It was possible to measure triple ionization of Kr and in applied studies the valence band of a laser excited dye-sensitized solar cell interface. The combination of the latter technique with transient absorption measurements is proposed.The organic molecule maleic anhydride (MA) and its binding configuration to the three anatase TiO2 crystals (101), (100), (001) has been investigated by means of Xray Photoelectron Spectroscopy (XPS) and Near Edge X-ray Absorption Fine structure Spectroscopy (NEXAFS). The results provide information on the binding configuration to the 101 crystal. High Kinetic Energy Photoelectron Spectroscopy was used to investigate multilayers of complexes of iron, ruthenium and osmium. The benefit of hard X-rays for ex-situ prepared samples is demonstrated together with the application of resonant valence band measurements to these molecules.
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| 4. |
- Hahlin, Maria, 1975-
(författare)
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Electronic and Molecular Surface Structures of Dye-Sensitized TiO2 Interfaces
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The dye-sensitized solar cell is a promising solar cell technology. In these systems the key process for light to electricity conversion is molecular in nature and is initiated in dye molecules adsorbed at a semiconducting surface. This thesis focuses on the electronic and molecular surface structure of the dye/TiO2 interface, and the experimental results were obtained from surface sensitive X-ray based electron spectroscopic methods. Two families of dyes, triarylamine based organic dyes and ruthenium based inorganic dyes, were investigated. The effect of dye structural modications on the interfacial properties was studied, such as the surface concentrations, dye molecular surface orientation, molecular interactions, and energy level matching. Also, the impact of additional parameters such as the incorporation of coadsorbents and the solvents used for dye sensitization were studied and complementary photoelectrochemical characterization was used to demonstrate functional properties corresponding to changes in the molecular layers. The experiments provided information on how specic structural modications change the frontier electronic structure. The results also showed that the adsorption of the organic dye leads to submolecular electronic changes, and that the dye surface orientations in general favor effcient energy conversion. Moreover, effects of solvents and coadsorbents, on both energy level matching between the dye and the TiO2 substrate and the surfacemolecular structure were quantied.
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