Untersuchungen zur Deckschichtbildung auf LiNi0,5Mn1,5O4-Hochvoltkathoden: Die Kathoden/Elektrolyt-Grenzfläche in Hochvolt-Lithium-Ionen-Batterien
Jülich / Forschungszentrum Jülich GmbH, Zentralbibliothek, Verlag (2017) [Book, Dissertation / PhD Thesis]
Page(s): 1 Online-Ressource (xvi, 158, xxxvi Seiten) : Illustrationen, Diagramme
In order to achieve the CO2 emission targets of the German Government, it is necessary to significantly reduce the CO2 emissions of passenger cars. The substitution of the conventional internal combustion engine by a battery electric drive is a suitable method for this purpose. For a high economical use of such vehicles, high range and low cost are of great importance. Both criteria can be achieved by the use of high-voltage cathode materials in the lithium ion batteries used. However, since these materials show a strong aging, their commercial use is not yet possible. The dominant aging mechanism in high-voltage cathode materials is not yet clearly described in the literature. In this work, the LiNi0.5Mn1.5O4 high-voltage material was used to identify this aging mechanism and to investigate its connection to the surface layer formation. Firstly, measurements were carried out on full cells, which had different lithium loadings. By means of the evaluation of the capacity profiles in combination with the method of differential capacity, the dominant aging mechanism could be identified as the loss of free lithium. This lithium is bound in the cell and is no longer available for further cell operation. In order to investigate whether the bound lithium is deposited as a topcoat on the electrode surface, a new method for sample preparation was developed. This results in very smooth sample surfaces and allows an optimal characterization of the surface layers on the battery electrodes. Visual examination of these samples by means of scanning electron microscopy showed the formation of a closed covering layer on the electrode. There are also indications that the surface layer is detached from the electrode surface again as a result of the state of charge. The characterization of the cover layer by means of X-ray photoelectron spectroscopy showed that this consists of decomposition products of the electrolyte and of lithium compounds. Studies on half and full cells showed, that the surface layers on electrodes of half-cells are significantly thicker than on electrodes of full cells. The magnitude of the surface layer formation thus depends on the lithium available in the cell.These studies provide a starting point to reduce the strong aging of the high-voltage cathode materials in lithium ion batteries and make this technology usable for commercial use.