Particle Architecture Regulates Anion Redox Reversibility via Stacking Fault Topology in Lithium‐Rich Cathodes

The utilization of reversible anion redox is essential for high‐energy lithium‐ion batteries, yet its fundamental coupling with particle architecture and crystallographic defects remains poorly understood. While single‐crystalline morphologies are widely recognized for mitigating mechanical degradation, their role in regulating electrochemical heterogeneity remains largely unexplored. Here, we demonstrate that the density and topology of planar defects specifically stacking faults are the primary determinants of anion redox reversibility in lithium‐ and manganese‐rich cathodes. By systematically comparing chemically invariant polycrystalline (high‐defect) and single‐crystalline (low‐defect) model systems, we reveal that stacking faults localized at grain interfaces act as catalytically active sites for excessive oxygen oxidation. This defect‐driven reactivity triggers irreversible phase transformations to rock‐salt structures, accelerates electrolyte decomposition, and drives severe voltage fade. Conversely, the suppression of such defects in single‐crystalline architectures preserves the integrity of the lattice oxygen, ensuring stable cycling. Our findings reframe the understanding of particle crystallinity from a mechanical descriptor to a critical electrochemical parameter, establishing defect‐topology engineering as a prerequisite for designing durable high‐energy cathode materials.