Gas evolution at graphite anodes depending on electrolyte water content and SEI quality studied by on-line electrochemical mass spectrometry

The gas evolution during the formation of graphite electrodes is quantified by On-line Electrochemical Mass Spectrometry (OEMS) for dry electrolyte (< 20 ppm H₂O) and 4000 ppm H₂O containing electrolyte to mimic the effect of trace water during the formation process. While the formation in dry electrolyte mainly shows ethylene (C₂H₄) from the reduction of ethylene carbonate (EC) and small amounts of hydrogen (H₂), the formation in water-containing electrolyte yields large amounts of H2 and considerable amounts of CO₂ in addition to the expected C₂H₄ evolution. We could show that a protective solid-electrolyte interphase (SEI) layer formed by pre-cycling the graphite electrode in 2% vinylene carbonate (VC) containing electrolyte can reduce the H2 evolution in watercontaining electrolyte by a factor of 7.5 compared to a pristine graphite electrode. Consequently, the ability of graphite electrodes to form an SEI prevents excessive gassing from trace water, which, e.g., is observed for non-SEI forming lithium titanate (LTO) electrodes.