For another Ni/Mn-based layered oxide, it was proposed that solvent molecules may enter the lattice when exceeding potentials of 4.0 V 79 . This would mean a larger contact area between the electrolyte and the active material, which might be the reason for a higher gas evolution. Also the P2 -O2 phase transition at about 4.15 V, which might also be a cause for higher reactivity . Both suggestions would be in line with our findings that show the strongest gas release at these potentials. It is known that lowering the upper cut-off potential can improve the cycle life of NaMNO-type electrodes significantly . Experiments with the upper cut-off at 3.8 V vs. Na + /Na confirm that the side reactions are mainly triggered at the high potentials. This potential is just at the onset to the high voltage plateau of the P2 -O2 phase transition. The results of these measurements are shown in Figure . Lowering the cut-off potential obviously leads to a much lower overall gas release, also at lower voltages, only the H2 signal during the first cycle of the 2G cell is higher. This, however, is in the range of deviation between similar cells. Figure SI-7in the Supporting Information compares two identically made 2G cells.