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

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 O2 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.