Primary SEI Formation on Lithium Metal – Influence of Film-Forming Additives

Lithium-metal batteries face more challenges than lithium-ion batteries (LIBs) with graphite-based negative electrodes. One significant challenge derives from the highly reactive nature of lithium metal, which can react with electrolytes upon contact to form the solid electrolyte interphase (SEI) and can develop detrimental dendrites upon cycling in contrast to graphite and silicon anodes, which require the application of current/voltage to form the SEI. Studies focusing on lithium metal SEI formation are less frequent than those for other anode materials. Through investigation of several electrolyte formulations including the film forming additives vinylene carbonate (VC) and fluoroethylene carbonate (FEC) we note multiple aspects of their respective SEI formation characteristics, as additives are consumed during interphase formation. Compared to VC, FEC shows a dependency on ethylene carbonate for equivalent interphase formation properties. Amounts of inorganic SEI compounds were found to be lowered after use of additives. Organic electrolyte-soluble compounds were not detectable if the additive is still present. This was achieved through an SEI accumulation process, leading to a sufficient analyte amount and complementary analytical methods ranging from quantitative gas analysis, over qualitative electrolyte analysis using NMR and mass spectrometry and solid residue analysis using XRD and quantitative gas chromatography.