Atomic-Scale\nand Nanoscale Structural Changes in LiCoO<sub>2</sub> Due to Irreversible\nDelithiation Accompanied by Oxygen Release

LiCoO<sub>2</sub> is a widely used cathode material in\nLi-ion batteries,\nbut its practical capacity is limited to about 60% of its theoretical\ncapacity. This limitation occurs because the reversible capacity significantly\ndecreases when more Li ions are extracted. Substantial delithiation\nleads to oxygen release, which has been regarded as a major factor\ncontributing to capacity fading. However, the associated microstructural\nchanges are not yet sufficiently understood, impeding a thorough comprehension\nof LiCoO<sub>2</sub> degradation stages. To investigate the final\ndegradation stage, we used scanning/transmission electron microscopy\n(S/TEM) to examine the microstructure of LiCoO<sub>2</sub> that had\nundergone irreversible defect formation due to near-surface complete\nLi removal by a NO<sub>2</sub>BF<sub>4</sub> oxidizer. Our results\nreveal several types of structural changes occurring sequentially:\noxygen release causes Co migration into Li sites, forming a CoO structure.\nThis leads to nanoscale pore formation and lattice expansion, which\nintroduces misfit dislocations at the interface between CoO and the\noriginal layered structure. These dislocations, pores, and CoO formation\nlikely disturb Li ion migration by blocking and distorting their pathways,\nand reducing the number of available Li sites. Consequently, these\nchanges are identified as atomic and nanoscale degradation factors\nin LiCoO<sub>2</sub>.