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It is possible to increase the charge capacity of transition-metal (TM) oxide cathodes in alkali-ion batteries by invoking redox reactions on the oxygen. However, oxygen loss often occurs. To explore what affects oxygen loss in oxygen redox materials, we have compared two analogous Na-ion cathodes, P2-Na_0.67Mg_0.28Mn_0.72O₂ and P2-Na_0.78Li_0.25Mn_0.75O₂. On charging to 4.5 V, >0.4e^– are removed from the oxide ions of these materials, but neither compound exhibits oxygen loss. Li is retained in P2-Na_0.78Li_0.25Mn_0.75O₂ but displaced from the TM to the alkali metal layers, showing that vacancies in the TM layers, which also occur in other oxygen redox compounds that exhibit oxygen loss such as Li[Li_0.2Ni_0.2Mn_0.6]O₂, are not a trigger for oxygen loss. On charging at 5 V, P2-Na_0.78Li_0.25Mn_0.75O₂ exhibits oxygen loss, whereas P2-Na_0.67Mg_0.28Mn_0.72O₂ does not. Under these conditions, both Na …