We could confirm in repeated measurements that a pressure increase within 800 h only takes place when NaClO 4 is present. The pressure inside the cell remains almost constant when propylene carbonate and Na metal are available. Additionally, it was observed that the propylene carbonate becomes a yellowish high-viscous compound over time. This effect results from polymerization reactions that take place between Na metal and PC. Such reactions were seen with and without NaClO 4 as conducting salt. When NaClO 4 is present, propylene oxide, as well as hydrogen and carbon monoxide, were detected in significant amounts (see also Figure in Reference 39 ). Additionally, we could see propene as well as propylene oxide in the MS gas measurement (Fig. ). The formation of propene, which was assigned by NIST data evaluation with high data match (940/1000), is proposed analogously for ethylene in EC-based electrolyte mixtures and could be confirmed in this study in case