Reversible Monoclinic Transition Buffering and Stepwise Uniform Delithiation in Ni-Rich Layered Cathodes

Ultrahigh-Ni layered oxides are promising high-energy cathodes but suffer from severe structural degradation during cycling, particularly due to irreversible phase transitions and interfacial instability. While extensive studies focus on the hexagonal (H) phase transition, the intermediate monoclinic (M) phase plays a critical yet overlooked role in mediating lattice strain and enabling ordered delithiation. Stabilizing the M phase remains challenging in ultrahigh-Ni cathodes due to linked bulk and surface degradation. Herein, we report stabilization of LiNiO₂ by combining Zr doping and Li₃NbO₄ coating, which respectively suppresses anisotropic lattice distortion by forming Li/Ni superlattices and provides uniform protection on primary particles. This dual-modification effectively inhibits NiO₆ bond length/angle dispersion within M phase, mitigating Jahn-Teller activity and establishing a reversible transition buffer that accommodates lattice strain and guides moderated M↔H conversion, thereby functioning as a structural bridge to ensure continuous symmetry recovery. The modified cathode achieves a stepwise uniform delithiation with layer-by-layer Li distribution, preventing separation into Li-sufficient and Li-deficient areas and delivering enhanced cycling stability with reduced voltage decay. This work demonstrates that M phase engineering is a promising strategy for developing ultrahigh-Ni cathodes with high energy density and prolonged stability.