Rotational stacking faults (RSFs) in layered oxide cathodes significantly contribute to electro-chemo-mechanical degradation, leading to microcrack formation and reduced cyclability. This study reveals that RSFs, arising from specific stacking sequences at different angles, play a critical role in determining structural and electrochemical stability. Combined experiments and calculations show that RSFs facilitate oxygen dimerization and transition-metal migration, promoting microcrack nucleation and propagation during cycling. Thermal defect annihilation can suppress RSFs, reducing microcracks and enhancing cyclability in lithium-rich layered cathodes. The presence of RSFs, previously overlooked, suggests a new synthetic guideline for high-energy-density layered oxide cathodes. The study highlights the complex interplay between electrochemical reactions and mechanical responses in lithium-ion battery electrodes, particularly in intercalation-based electrodes. Repeated ion (de)intercalation processes cause cumulative fatigue and mechanical stress, leading to electro-chemo-mechanical degradation. In conventional polycrystalline particles, failure modes manifest as intergranular and intragranular cracks. However, even single-crystalline cathodes undergo consistent degradation, indicating the need to revisit the role of intragranular cracks. Intriguingly, intragranular cracks in layered structures form in planar features, propagating along crystallographic ab-planes. These cracks are predominantly produced under high-voltage operations (>4.5 V vs. Li⁺/Li), causing gliding of layered planes along (001) planes. This gliding is initiated by phase transitions among different symmetrical stackings, contingent on the composition of the mother phase. The presence of RSFs, which are intrinsic defects in layered materials, can lead to anisotropic stress fields during gliding processes. The study revisits the electrochemo-mechanical degradation mechanism of high-energy-density layered oxides, providing evidence that RSFs are key factors in the premature formation and propagation of internal cracks. Using single-crystalline O2-type lithium-rich layered oxide as a model system, the study demonstrates that RSFs can be identified when multiple stacking sequences, which should be observed at certain rotation angles, are detected along the 0° projection angle. Density functional theory (DFT) calculations show that RSFs cause TM slabs to glide independently, shortening bond lengths between oxidized oxygens during charging. Ab initio molecular dynamics (AIMD) simulations reveal an autocatalytic behavior, where increased TM migration near RSFs regions accelerates the formation of short oxygen dimers, leading to intragranular crack formation. Scanning transmission electron microscopy (STEM) measurements, combined with geometric phase analysis (GPA), confirm the presence of RSFs and their role in mechanical degradation. Thermal annihilation of RSFs was validated by preparing electrodes with varying RSF levels and systematically annealing them. Annealing at higher temperatures reduced RSFs, leadingRotational stacking faults (RSFs) in layered oxide cathodes significantly contribute to electro-chemo-mechanical degradation, leading to microcrack formation and reduced cyclability. This study reveals that RSFs, arising from specific stacking sequences at different angles, play a critical role in determining structural and electrochemical stability. Combined experiments and calculations show that RSFs facilitate oxygen dimerization and transition-metal migration, promoting microcrack nucleation and propagation during cycling. Thermal defect annihilation can suppress RSFs, reducing microcracks and enhancing cyclability in lithium-rich layered cathodes. The presence of RSFs, previously overlooked, suggests a new synthetic guideline for high-energy-density layered oxide cathodes. The study highlights the complex interplay between electrochemical reactions and mechanical responses in lithium-ion battery electrodes, particularly in intercalation-based electrodes. Repeated ion (de)intercalation processes cause cumulative fatigue and mechanical stress, leading to electro-chemo-mechanical degradation. In conventional polycrystalline particles, failure modes manifest as intergranular and intragranular cracks. However, even single-crystalline cathodes undergo consistent degradation, indicating the need to revisit the role of intragranular cracks. Intriguingly, intragranular cracks in layered structures form in planar features, propagating along crystallographic ab-planes. These cracks are predominantly produced under high-voltage operations (>4.5 V vs. Li⁺/Li), causing gliding of layered planes along (001) planes. This gliding is initiated by phase transitions among different symmetrical stackings, contingent on the composition of the mother phase. The presence of RSFs, which are intrinsic defects in layered materials, can lead to anisotropic stress fields during gliding processes. The study revisits the electrochemo-mechanical degradation mechanism of high-energy-density layered oxides, providing evidence that RSFs are key factors in the premature formation and propagation of internal cracks. Using single-crystalline O2-type lithium-rich layered oxide as a model system, the study demonstrates that RSFs can be identified when multiple stacking sequences, which should be observed at certain rotation angles, are detected along the 0° projection angle. Density functional theory (DFT) calculations show that RSFs cause TM slabs to glide independently, shortening bond lengths between oxidized oxygens during charging. Ab initio molecular dynamics (AIMD) simulations reveal an autocatalytic behavior, where increased TM migration near RSFs regions accelerates the formation of short oxygen dimers, leading to intragranular crack formation. Scanning transmission electron microscopy (STEM) measurements, combined with geometric phase analysis (GPA), confirm the presence of RSFs and their role in mechanical degradation. Thermal annihilation of RSFs was validated by preparing electrodes with varying RSF levels and systematically annealing them. Annealing at higher temperatures reduced RSFs, leading