Detection and charackerization of Lithium plating

Abstract

Lithium plating is not only the most severe ageing mechanism in lithium-ion batteries (LIBs) but also becoming more and more important due the increasing presence of electric vehicles (EVs). In EVs the extreme conditions causing lithium plating, like very high charging currents and low environment temperatures, are much more prevalent than in consumer electronics. Due to the high number of factors that influence the plating process, ranging from the cell geometry to the chemical composition of the electrolyte, a deeper understanding of the plating process is still lacking. Without this knowledge it is hard to design cells in a plating resistant way, or to operate cells under the ideal conditions to minimize plating. This thesis aims at showing different methods to investigate the plating process on three different levels. The first method is on the cell level, investigating the behaviour of the whole cell during plating. It contains the analysis of the voltage and current profiles that show an atypical behaviour during plating. The focus of the analysis is on the current profile of the constant voltage (CV) phase during charging under low temperature conditions leading to plating. This current profile can be fitted with the Johnson-Mehl-Avrami-Kolmogorov (JMAK) function that describes the electrochemical deposition process of a metallic species on a surface. The resulting fitting parameters can be utilized to characterize the plating behaviour of the cell as well as better estimate the amount of plated lithium than commonly used methods. It can also potentially predict the future safety risk due to dendrite formation. In the second part the chemical composition of the surface electrolyte interface (SEI) is investigated using X-ray photoelectron spectroscopy (XPS). The composition as well as the mechanical properties of the SEI are strongly influencing the plating process and preliminary work has shown that plating is also changing the morphology of the SEI and increasing its thickness drastically. Cells under different conditions (plated, charged and discharged) as well as cells of different manufacturers have been probed using XPS. During the measurements an unwanted side effect of the experimental setup was discovered that lead to a migration of lithium to the surface of the sample and was distorting the measurement results. Regardless of the effect, it was possible to see that the SEI can have a very different composition in cells of different manufacturers and that plating not only changes the morphology but also the composition of the SEI. The unwanted side effect could furthermore be utilized to identify samples that were plated recently and could be used in further more controlled experiments to localize lithium depositions on plated samples. In the last part the particle structure of the anode surface of cells of different manufacturers was investigated using a watershed particle detection algorithm on laser scanning microscopy (LSM) images of the anode surfaces. The distributions of the particle sizes have then been compared to the capacity loss in plated cells. It was shown that the capacity loss correlates with parameters extracted from the particle size distributions. It is however necessary to create more data to verify this correlation. In summary this thesis utilized new methods to detect or characterize plating on different levels of magnification, from the cell level to the chemical composition. New approaches were found to predict a cells future plating behaviour, spatially localize plated areas on the anode and design cells in a plating resistant way.