In the case of PC, the propene signal shows a small peak at high potentials together with the first sharp increase in CO2. The peak is absent in the following cycles, but the propene signal slightly increases over cycling and shows minima together with the CO2 release. Such a first peak of alkene formation is usually attributed to the SEI formation at the anode, here Na metal . Interestingly, no such cycling dependent behavior was observed in the symmetric Na| Na cell, but a more constant decay in the beginning of the measurement. Propene is known to be the product of the ring-opening reaction from cyclic carbonates. This process is catalyzed in presence of trace water and even stronger OH -under battery operating conditions . Since the peak occurs together with CO2 evolution, which we attribute to the decomposition of inorganic carbonates with protic species, we suggest that this initial propene peak is caused by ring-opening of PC under these protic conditions.