Determination of Degradation Mechanisms During the Cyclic Ageing of Li-ion Batteries

Kurzfassung

Degradation mechanisms in commercial Li-ion batteries have been extensively studied over the past decades. However, there are still open questions regarding the link between the observed changes on the system level of the battery with the actual degradation on the material level happening inside the battery. Therefore, different techniques are applied and findings combined to improve the understanding of material degradation influences on the system level behavior. A tailored electrochemical strain microscopy (t-ESM) technique, which is based on atomic force microscopy, is presented. The technique is applied to laboratory made silicon composite anodes and commercial LiFePO4 cathodes. The surface displacement induced by the Vegard strain is proportional to the ionic concentration change and activity of Li-ions within the probed volume under the tip. The probed volume spans only a few nanometers in depth inside the material. The results indicate a structural dependency, with higher mobility and activity at boundaries. The technique provides time constants and hence diffusion coefficients on the nano-scale. The electrochemical activity, which is analyzed with the t-ESM technique, is found to decrease due to ageing. The ageing of commercial battery cells is monitored using system level methods such as incremental capacity analysis (ICA), differential voltage analysis (DVA) and tracking of the open-circuit voltage (OCV). Ageing at 50% depth of discharge (DOD) leads to higher capacity fading compared to cycling over 100% of the available capacity. In total, 14 different features of the cell behavior are tracked during the ageing process using various analysis techniques. The intensity of the first discharge peak of the ICA curves is found to correlate well with the capacity fading and is used to predict the remaining capacity and lifetime. The loss of lithium and the loss of active material, especially at the cathode, are the main factors leading to the observed capacity decay. The post-mortem analysis reveals the formation of lithium fluoride crystals on the anode surfaces, aged at 55°C. The iron dissolution (from the electrode into the electrolyte) is shown to be temperature dependent and in addition dependent on the depth of discharge. Next to iron dissolution, vanadium dissolution (from the electrode into the electrolyte) from the cathode is observed. Micrometer sized agglomerates (consisting out of nano particles from cathode active material) are observed on the cathode surface, which decrease the surface conductivity, but does not directly correlate with the capacity fading. Ageing at -20°C leads to the formation of stable Li plating, which is subsequently observed at room temperature cycling.