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The cathode in rechargeable lithium-ion batteries operates by conventional intercalation; Li⁺ is extracted from LiCoO₂ on charging accompanied by oxidation of Co³⁺ to Co⁴⁺; the process is reversed on discharge. In contrast, Li⁺ may be extracted from Mn⁴⁺-based solids, e.g., Li₂MnO₃, without oxidation of Mn⁴⁺. A mechanism involving simultaneous Li and O removal is often proposed. Here, we demonstrate directly, by in situ differential electrochemical mass spectrometry (DEMS), that O₂ is evolved from such Mn⁴⁺-containing compounds, Li[Ni_0.2Li_0.2Mn_0.6]O₂, on charging and using powder neutron diffraction show that O loss from the surface is accompanied by diffusion of transition metal ions from surface to bulk where they occupy vacancies created by Li removal. The composition of the compound moves toward MO₂. Understanding such unconventional Li extraction is important because Li−Mn−Ni−O …