Atomic Origins of Ultrahigh-Voltage Failure in LiCoO2 Cathodes.

Pushing lithium cobalt oxide (LCoO2) toward extremely high-voltage operation up to 5V is critical to boosting a battery's energy density for future compact electronics. However, its degradation mechanisms at such extreme voltages remain unexplored. Here, we employ machine-learning-aided super-resolution electron microscopy to directly visualize, at atomic resolution, how LCoO2 structurally deteriorates under 5 V for the first time. We discover that deep delithiation activates global deformation in which in-plane shear breaks the O3 lattice into nanoscale mosaics of O1 and reoriented O3 domains, while out-of-plane distortions drive cracking and kinetically trapped structural motifs. Upon extended cycling, these coupled chemomechanical processes evolve into a frustrated surface architecture comprising intertwined misoriented domains and antiphase boundaries. Building on these mechanistic insights, we deliver a proof-of-concept demonstration that rationally designed codoping provides a targeted route to mitigate the coupled deformation and phase-degradation cascade, markedly pushing the cycling stability of LiCoO2 toward unprecedented ultrahigh voltage. Our work establishes a new paradigm for materials optimization by leveraging atomic-scale diagnostics to mitigate degradation at its origin.