Lithium metal negative electrode: metallurgical and electrochemical aspects

For electric vehicle applications, the use of lithium (Li) metal as negative electrode is very promising because Li has the highest theoretical capacity and a low electrochemical potential. Unfortunately, stripping and plating heterogeneities lead to dendrite growth on Li electrode surface which induces short-circuits. The use of a solid polymer electrolyte is one of the possible solutions to mitigate dendrite growth. However, batteries still have a limited cycle life whose failure is related to Li metal negative electrode.
The challenge of this PhD was to understand the mechanisms at stake in the Li metal electrode causing battery end-of-life. Particular attention was paid to correlate Li metallurgy and its impact on battery failure. Using X-ray computed tomography, the samples were analyzed in a non-intrusive manner while avoiding contact with air and moisture.
A first study focused on the characterization of Li metal microstructure (precipitates, Li grain size and inclusions). Thanks to the insights provided by this study, the evolution of Li metal morphology during cycling could be followed in two types of assemblies (Li symmetric cells and batteries) and correlated with Li microstructure. The Li symmetric cells allow, thanks to a polarization of a few days or weeks, to characterize the impact of Li oxidation and reduction on Li electrode morphology. The study of batteries, requiring several months of cycling, allows to observe the degradation of the Li metal electrode upon cycling and to deepen the mechanisms of battery failure.