Aktuelle Dissertationen

"Local structural changes in cathode materials for rechargeable Li-ion batteries studied using X-ray Absorption Spectroscopy"

Jatinkumar K. Rana

Local structural changes in the Li2MnO3, LiMn0.4Ni0.4Co0.2O2, 0.5 Li2MnO3 · 0.5 LiCoO2 and 0.5 Li2MnO3 · 0.5 LiMn0.4Ni0.4Co0.2O2 cathodes are investigated using the X-ray ab­sorption spectroscopy (XAS). Qualitative information about the average valence state of the absorbing atom and changes in the local coordination symmetry are estimated from the near-edge region (XANES) of the absorption spectra, while geometrical changes in the local structure around the absorbing atom are quantified by fitting the extended region of the absorption spectra (EXAFS) with theoretical models. By fitting the EXAFS data, I provide direct evidence for the charge compensation by each of the transition metal (TM) ions upon lithium extraction and/or re-insertion and associated structural changes. The electrochemical processes in the Li2MnO3 cathode are complex and non-conventional. Both oxygen-loss and ion-exchange mechanisms play a significant role during the activa­tion of the cathode. Lithium extraction occurs with the concurrent removal of oxygen, giving rise to the formation of MnO2-type structure (R-3m), while the presence of protons in the interslab region, as a result of Li+-H+ exchange, alters the stacking sequence of oxygen layers from O3-type to P3-type. Lithium re-insertion reverts the P3-type stacking sequence back to the O3-type, giving rise to formation of Li2MnO3-type structure which is oxygen deficient. Oxygen removal occurs only during the activation of material and at slower rates. Upon subsequent cycling, charge compensation occurs by the Li+ ↔ H+ exchange only, with the structural flip-over between P3-type ↔ O3-type. The repetitive changes in the stacking sequence during each cycle are responsible for the structural degradation, and in turn fading electrochemical performance of Li2MnO3 upon cycling. Lithium extraction/re-insertion from/into the LiMn0.4Ni0.4Co0.2O2 cathode occurs in the conventional way via oxidation/reduction of its TM ions. However, deep delithiation re­sults in an irreversible transition of the O3 structure to the O1-type structure. There is no solubility between the two components of the composite 0.5 Li2MnO3 · 0.5 LiCoO2 cathode. However, there are some indications of solubility between two components of the 0.5 Li2MnO3 · 0.5 LiMn0.4Ni0.4Co0.2O2 cathode. Both components of the composite cathode respond to the electrochemical activation in their own unique ways. Li2MnO3 domains act as a source of excess lithium above 4.4 V and protects LiMO2 (M=Mn, Ni or Co) component from complete delithiation and deteriorating structural changes.

"Analysis of diffraction line-broadening and diffraction-line shifts by RIETVELD refinement of energy-dispersive synchrotron diffraction patterns"

Fig.: Result of the Rietveld-refinement of
an energy-dispersive diffraction pattern of a
CeO2 powder sample, recorded under 2θ = 10°.

Daniel Apel

The energy-dispersive diffraction patterns contain a variety of information about the structural material properties. However, until now the wealth of information included in energy-dispersive diffraction patterns has not been fully accessed. To exploit the amount of provided information and also to use the essential advantage of the method compared with angle-dispersive diffraction, which lies in the fast measurement of complete diffraction patterns in well-defined but arbitrary scattering directions, a Rietveld refinement program code has been developed to analyze energy-dispersive diffraction data. The aim of this project is to handle energy-dispersive diffraction data sets measured in various orientations (j,y) in such a way that models describing the depth dependence of the phase specific residual stresses, of domain size as well as of microstrain can be refined simultaneously. To analyze line-broadening observed in the energy-dispersive diffraction patterns, evaluation procedures applied so far in angle-dispersive diffraction were adapted to the energy-dispersive case. A model to separate size and strain broadening is introduced. It is shown that even though the instrumental resolution of the energy-dispersive diffraction is inferior compared to the most angle-dispersive diffraction instruments, line-broadening assignable to microstructural effects can still be observed. Concerning the evaluation of macro residual stress depth gradients in the near-surface region of quasi-isotropic polycrystalline materials by means of Rietveld refinement, the formalism is applied to mechanically surface treated samples with well-defined in-plane residual stress distributions within the accessible information depth of the X-rays. The results are compared with those obtained by means of the “real space” and the conventional “Laplace space” methods.

Skalen- und methodenübergreifende Analyse randschichtnaher Eigenspannungsverteilungen in vielkristallinen Werkstoffen und Bauteilen

Tillman Fuß