The use of lithium metal as an anode material offers the potential of a higher energy density compared to standard graphite intercalation anodes and is therefore an interesting option for future batteries. When using lithium metal with standard liquid electrolyte systems from state-of-the-art lithium-ion batteries, a decreased performance and stability is observed, caused by strong SEI formation and dendrite-growth. Both, growth of SEI and of lithium dendrites as well as additional changes of the morphology of the lithium surface have a crucial influence on the voltage and impedance behavior the battery cell. The growth of Li-dendrites may even lead to short circuits and thus to failure of the battery. Accordingly, it is important to investigate and gain understanding of these impact factors to ensure safe and efficient operation of batteries based on lithium metal anodes.
For the characterization of the mentioned effects and influences, electrochemical impedance spectroscopy (EIS) is a reliable and well-established method. It facilitates distinguishing different physical processes within the battery based on their frequency ranges. Through the use of experimental cells, it is possible to extend the detail of the analysis down to the half-cell level. The experimental cells also enable analysis of so called symmetric cells made up of two lithium metal anodes, excluding distortion of the results by cathode effects.
In this contribution, we will present a characterization methodology with experimental cells for lithium metal anodes and results of an investigations with different liquid electrolyte systems and different anode materials. A total of six different electrolytes were investigated, following different variations, like for example different solvents or conductive additives. The investigations of the impedance and voltage behaviour were carried out in a symmetrical setup. In addition to the characteristics of pristine cells, the cycle stability as well as the impact of different current profiles were investigated. The results of the various material configurations are compared over the entire characterization process in order to get a general understanding of the results and the underlying processes.