The two overlapping are one at the beginning of the high voltage plateau and one at the upper cut-off voltage. This behavior can be seen for other materials as well . An explanation for the first rise during the voltage step might be the cracking of the material generating a higher surface area , while the second should be more related to active lattice oxygen that oxidizes the electrolyte. It was shown that similar materials than the here presented, Na0.66Ni0.33Mn0.67O2, undergo anionic oxygen redox and show oxygen release at potentials over 4.2 V 77 . It is known that alkali metal rich layered oxides can undergo oxygen release at high potentials . No evidence for oxygen release from the active material was found in the MS data. However, for some active materials, there might be the possibility that reactive oxygen species react with the electrolyte or the conductive additive to form CO2, which is then detected instead by the MS . This was shown with O-labeled cathode materials of similar structure by House et al. While the lattice oxygen in P2-Na0.67Mg0.28Mn0.72O2 appeared stable, reactions involving lattice oxygen were found for the Li-stabilized layered oxide P2-Na0.78Li0.25Mn0.75O2 .