Untersuchung der Alterung von Lithium-Ionen-Batterien mittels Elektroanalytik und elektrochemischer Impedanzspektroskopie

The use of electrochemical impedance spectroscopy for energy storage systems can be compared with the recording of an electrocardiogram (EKG) of the heart. It is not possible to understand the functionality of the heart by means of an EKG alone. With the basic understanding of its function and the associated signals, the EKG is nevertheless one of the most important procedures for diagnosing the cardiac function. Likewise, electrochemical impedance spectroscopy (EIS) has the potential to diagnose a variety of aging effects and malfunctions in electrochemical energy storage.Though some parameters are correlated with aging effects, it is found that there is a great deal of disagreement in the literature regarding the effect of aging effects onto the electrochemical impedance spectrum. It is shown in this work that the model-based coupling of comparatively simple physical processes can already produce a large number of phenomena in the EIS. Thus it is difficult to perform an unambiguous assignment of physical processes to the observed electrochemical impedance spectra.However, as long as the basic understanding of the effects of individual aging processes on the EIS is unknown, its possibilities are severely limited. This work takes some steps towards a more complete description of aging effects in lithium-ion batteries and also on the EIS in combination with other electroanalytical procedures. It is a comprehensive study of the electrochemical properties in various aging conditions which can be measured without destruction of the cell. Therefore common methods such as the method of distributed relaxation times (DRT) and also new methods using differential spectra (D-EIS) are used. The effects of aging on processes with slow time constants are investigated by the differential voltage analysis (DVA) of the discharge curves .In the experimental part of this thesis a complex test matrix is evaluated, which shows the dependence of the aging of the cells from different depths of discharge and mean states of charge, covering a test duration of several years. The results of this study, the changes in electrochemical properties, are checked for their transferability to another cell type. Thereby also half-cells and microscopic methods are used to describe the electrochemistry of individual electrodes