Lithium ion batteries are complex multi-component systems. The large variety of materials available for each part of the battery cell and the reactions between them determine the high complexity of these batteries. Early studies already pointed out the importance of chemical and electrochemical reactions in lithium ion batteries. In contrast to most of the other battery technologies, lithium ion batteries operate outside of the electrochemical stability window of the electrolyte. This leads to a cascade of reactions strongly affecting the properties of the battery cell. The electrolyte does not only undergo electrochemical reactions in contact with the electrodes but also chemically induced degradation reactions. These reactions are under investigation since the beginning of lithium ion battery research.

Within early studies, electrolyte samples extracted from a battery or prepared outside of a battery were analyzed to identify newly formed reaction products. Numerous compounds were found as degradation products of common lithium ion battery electrolytes. However, identifying these compounds was not sufficient to conclude and characterize reaction pathways and to determine the impact on the properties of the battery cell. A major challenge of chemical analyses of lithium ion battery electrolytes is a very high reactivity, in particular sensitivity to hydrolysis of specific reaction products. Therefore, analysis methods requiring an extraction of the electrolyte involve a high risk of sample alterations potentially leading to misinterpretations.

Within this presentation, we will show how we used straightforward ex situ experiments to investigate single reaction steps and how we developed sophisticated in situ techniques to put together the single steps to finally enable the assembling of a whole picture of the reactions in a lithium ion battery. This work includes, for example, NMR spectroscopy and gas chromatography.