Advancing and Understanding Lithium-Ion Differential Thermal Analysis

Abstract

Li-ion differential thermal analysis is the only method now available that can probe the phase composition of the electrolyte directly and reliably in a Li-ion cell. While there are a variety of methods that can be used to determine the state of an electrolyte that has been extracted from a Li-ion cell, all such methods require complete destruction of the cell. The extraction process itself carries the risk of incongruence between the extracted sample and the electrolyte as it existed in the cell. Further, traditional methods involve direct handling of electrolyte samples, which introduces risks of mishandling and experimental error. Li-ion differential thermal analysis is the only method of analyzing the phase composition of an electrolyte that allows for repeated testing of a cell, does not damage the cycling capacity of the cell, and can provide robust data on the state of the electrolyte. As this thesis will define and describe for future researchers, there are several limitations to the broad application of this technology. Despite these limitations, this thesis details several applications that cannot be matched by other cell testing techniques. The first is a method to rank the tortuosities of negative electrodes. The second is a simple way to test whether significant electrolyte change has occurred during long term cell testing that is otherwise difficult to observe. The third is a check to determine if conventional methods of electrolyte analysis have produced accurate results. The fourth is a model that can predict the liquidus point of an electrolyte. Overall, Li-ion differential thermal analysis is shown to be an excellent tool for the in situ study of electrolytes in Li-ion cells.