Ni Migration-Induced Strain and Phase Segregation in LiNiO₂ Cathodes

High-nickel layered oxides are promising cathode materials for next-generation lithium-ion batteries due to their high energy density, but their structural instability at high voltages remains a critical challenge. This study investigates how Ni migration influences phase segregation, Li/Ni antisite defect accumulation, and mechanical degradation in LiNiO₂ (LNO) through a combination of synchrotron X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS), transmission electron microscopy (TEM), and density functional theory (DFT) calculations. The results reveal that at high voltages, Ni migrates into tetrahedral sites, leading to the formation of coexisting <i>R</i>3̅<i>m</i> phases with distinct lattice behaviors. This phase segregation induces inhomogeneous lithium distribution, internal stress, and strain accumulation, ultimately promoting microcrack formation and accelerating structural degradation. While Ni migration is partially reversible upon discharge, continuous cycling leads to the progressive accumulation of Li/Ni antisite defects, which disrupt Li-ion transport and further destabilize the structure. The findings describe the role of bulk Ni migration in phase segregation, linking atomic-scale lattice distortions to long-term performance limitations in high-Ni cathodes. These insights suggest strategies such as selective doping to mitigate Ni migration and improve the structural stability.