Resolving the Structural Origin of Lattice Oxygen Release across Scales in Practical Lithium‐Manganese‐Rich Electrodes

Gas evolution from lattice oxygen release is a major obstacle to deployment of high‐energy‐density cathodes, yet its structural origin within dense electrodes remains challenging to resolve across multiple length scales. This difficulty arises because gas evolution is the collective outcome of oxygen release from numerous grains and particles, whose environments can vary across electrode, secondary‐particle, and grain scales. Here, by integrating online electrochemical mass spectrometry with synchrotron X‐ray and electron microscopy, this study resolves the spatial origin of O 2 evolution through mapping Mn reduction, a chemical fingerprint of oxygen release, across four orders of magnitude in length scale. Two lithium‐manganese‐rich layered oxide (LMR) electrodes that vary only in secondary‐particle size are employed as a model system, and their O 2 evolution differs twofold. Spatially resolving Mn oxidation state distribution reveals no detectable Mn‐reduction gradient across the electrode thickness or within particles, indicating that the difference does not originate from electrode‐ or particle‐scale heterogeneity. Instead, frontal facets (lithium‐intercalation planes) of grains exhibit deeper Mn reduction and surface reconstruction than lateral facets, consistent with favorable oxygen vacancy formation and migration. O 2 evolution matches after frontal‐facet area normalization, identifying frontal‐facet exposure as a key structural descriptor for O 2 evolution in these practical LMR electrodes.