For Na0.58Ni1/3Mn2/3O2, Cai et al. determined 2.6 -3.8 V as stable voltage window and suggested that higher voltages cause side reactions due to the formation of O2 x-species . This is in good agreement with what was found in the here-studied Mn-richer material. Even if no O2 release was found, the reactive oxygen species are likely be the reason for increased electrolyte oxidation at these potentials.