Clarifying the origin of molecular O₂ in cathode oxides

Anionic redox has reshaped the conventional way of exploring advanced cathode materials for Li-ion batteries. However, how anions participate in the redox process has been the subject of intensive debate, evolving from electron holes to O-O dimerization and currently to a focus on trapped molecular O₂ based on high-resolution resonant X-ray inelastic scattering research. Here we show that the resonant X-ray inelastic scattering signal of molecular O₂ is not exclusive to Li-rich oxide cathodes, but appears consistently in O-redox-inactive oxide materials even with a short beam exposure time as low as 1 min, indicating that molecular O₂ species are not directly related to voltage hysteresis and voltage decay. We further demonstrated that molecular O₂ is not a direct product of electrochemistry but more likely a consequence of the core excitation process in resonant X-ray inelastic scattering, for which the possible scenarios of the dissociation of 'M-(O-O)'-like species on beam excitation must be considered. Collectively, our results reconcile the conflicting reported results on the (non-)observation of molecular O₂ signal collected from different beamlines and suggest that molecular O₂ is not the energetic engine of new battery oxide cathodes.