Regulating Oxygen‐Mediated Reconstruction during Cooling: A New Design Principle for High‐Capacity Sodium Manganese Oxide Cathodes

ABSTRACT Deciphering the synthetic origin of oxygen‐redox stability is critical to the design of high‐capacity sodium‐ion cathodes. Although prior studies have correlated electrochemical behavior with pre‐formed structural motifs, the pathways by which these motifs emerge during synthesis remain unclear. Here, we track the structural evolution of layered sodium manganese oxide during solid‐state synthesis using time‐resolved X‐ray diffraction coupled with a real‐time micro‐furnace. We show that oxygen uptake and Mn oxidation occur predominantly during cooling, triggering oxygen‐mediated reconstruction into a framework enriched with manganese vacancies (□Mn) and stacking faults. By kinetically regulating the cooling process, we reveal that slow cooling promotes a spatially homogeneous stacking‐fault‐rich architecture that suppresses in‐plane Mn migration and mitigates irreversible O 2 release. High‐resolution microscopy directly visualizes the development of stacking faults, while density functional theory shows that these defects substantially increase the barrier for Mn migration. Together, these findings identify cooling‐driven oxygen‐mediated reconstruction as a previously underappreciated synthetic lever for defect engineering and establish a general design principle for stabilizing oxygen redox in layered sodium cathodes.