NMR as a powerful tool to study lithium ion battery electrolytes

The properties of a lithium ion battery (LIB) are determined by the interplay of its components. In this regard, it is essential to understand the interactive behaviour of the electrolyte as it interacts with both the positive and the negative electrode as well as with the separator and other inactive cell components. The behaviour of LIB electrolytes including their chemical and electrochemical reactions can be monitored by means of numerous analytical techniques. NMR spectroscopy demonstrated to be one of the most powerful analysis methods. Analysing LIB electrolytes appeared very challenging, as they generate many reaction products that are highly reactive, particularly in the presence of water. In this regard, NMR spectroscopy proved itself valuable since it enables contact-free measurements of samples inside protective vessels. This review describes the progress in analysing LIB electrolytes starting from qualitative studies on the formation of degradation products. Additionally, it includes early quantitative studies to gain deeper insights into chemical reactions as well as the development of new NMR methods providing results that partially revise previous works. Finally, in situ NMR methods that enable the analysis of electrolytes inside of an intact battery cell are discussed.