Atomic-Scale and Nanoscale Structural Changes in LiCoO₂ Due to Irreversible Delithiation Accompanied by Oxygen Release

LiCoO2 is a widely used cathode material in Li-ion batteries, but its practical capacity is limited to about 60% of its theoretical capacity. This limitation occurs because the reversible capacity significantly decreases when more Li ions are extracted. Substantial delithiation leads to oxygen release, which has been regarded as a major factor contributing to capacity fading. However, the associated microstructural changes are not yet sufficiently understood, impeding a thorough comprehension of LiCoO2 degradation stages. To investigate the final degradation stage, we used scanning/transmission electron microscopy (S/TEM) to examine the microstructure of LiCoO2 that had undergone irreversible defect formation due to near-surface complete Li removal by a NO2BF4 oxidizer. Our results reveal several types of structural changes occurring sequentially: oxygen release causes Co migration into Li sites, forming a CoO structure. This leads to nanoscale pore formation and lattice expansion, which introduces misfit dislocations at the interface between CoO and the original layered structure. These dislocations, pores, and CoO formation likely disturb Li ion migration by blocking and distorting their pathways, and reducing the number of available Li sites. Consequently, these changes are identified as atomic and nanoscale degradation factors in LiCoO2.