Atomic-Scale Imaging of Li+ Trapping at Defects in Degraded LiCoO₂

Layered lithium cobalt oxide (LiCoO₂, LCO) serves as a pivotal cathode material in portable electronics. However, charging beyond 4.2 V triggers progressively irreversible phase transitions and structural collapse in LCO, leading to capacity fading. Unraveling the degradation mechanism necessitates direct atomic-scale characterization of the defect structures and the quantitative determination of the atomic configurations of lithium, which remains a formidable challenge. Herein, we precisely determine the atomic structures of LCO charged to 4.3 V via multislice electron ptychography (MEP) that enables simultaneous and quantitative imaging of Li, O, and Co with depth resolution. The delithiation level quantified from the MEP-reconstructed images aligns well with the macroscopic electrochemical measurements. Further examination of the intragranular microcracks formed in charged LCO upon cycling reveals that the defect evolution is accompanied by Co dissolution more extensive than O release, along with increased curvature of Li⁺ diffusion channels and Li/Co antisite mixing at the defect core. Moreover, we directly quantify that 16% of Li⁺ is trapped within a ∼2 nm region near the defect core, where a 23% Li⁺ concentration difference is observed over a 10 nm depth. These phenomena are closely associated with Co/O deficiency and CoO₆ octahedra distortions. Our work provides direct atomic scale evidence of the electrochemical degradation and underscores the broad potential of MEP across diverse Li⁺ battery systems.