This study investigates the role of rotational stacking faults (RSFs) in the electro-chemo-mechanical degradation of layered oxide cathodes, particularly in lithium-rich layered oxides. RSFs, which occur when layers are misaligned at specific angles, are found to facilitate oxygen dimerization and transition-metal (TM) migration, leading to microcrack nucleation and propagation. These microcracks contribute to the cumulative electro-chemo-mechanical degradation during cycling. The authors demonstrate that thermal defect annihilation can suppress RSFs, reducing microcracks and enhancing the cyclability of lithium-rich layered cathodes. The findings highlight the importance of managing RSFs to improve the long-term stability and performance of high-energy-density layered oxide cathodes.This study investigates the role of rotational stacking faults (RSFs) in the electro-chemo-mechanical degradation of layered oxide cathodes, particularly in lithium-rich layered oxides. RSFs, which occur when layers are misaligned at specific angles, are found to facilitate oxygen dimerization and transition-metal (TM) migration, leading to microcrack nucleation and propagation. These microcracks contribute to the cumulative electro-chemo-mechanical degradation during cycling. The authors demonstrate that thermal defect annihilation can suppress RSFs, reducing microcracks and enhancing the cyclability of lithium-rich layered cathodes. The findings highlight the importance of managing RSFs to improve the long-term stability and performance of high-energy-density layered oxide cathodes.