The material shows a solid solution behavior in the voltage window of 1.5 -4.0 V, with a single P2 phase. At 4.2 V it shows a voltage plateau and a two-phase reaction with an O2 phase. It was shown before, that this high voltage plateau is mainly responsible for observed capacity fade of the material. This suggests side reactions to be dominating in this state of charge, which can be detected by the here presented DEMS data. A detailed description of the NaMNO material can be found in a recent publication , which focuses on the material synthesis and its characterization. The measurements shown in Figure were made using the 1M NaPF6 in 2G (left) and PC (right), in a voltage window of 1.5 V to 4.25 V vs. Na + /Na reference. H2 as main gas: For both electrolytes, the main gas formed is H2 although the total amount is much higher in the case of PC. The formation of H2 in DEMS measurements can be caused by a number of factors . Trace water impurities are a common reason but for several reasons we consider this unlikely to be the main source. All cell parts were carefully dried and any remaining water traces should have reacted with the Na metal prior to the measurements as the cells equilibrated over 24 hours. Trace water should also continuously react with Na metal, resulting in continuous H2 evolution . This is obviously not the case as the H2 evolution changes over cycling. A further indicator is the absence of signals for POF3, which is a common product of the hydrolysis of PF6 -. Taken together, these considerations strongly suggest that trace water is not the dominant source of H2, but instead originates from electrolyte solvent degradation .