Surface Reconstruction as a Design Principle for Ni-rich Cathodes

Surface reconstruction by formation of inert phases in Ni-rich cathodes is widely viewed as a degradation mechanism for batteries. Herein, this seemingly undesirable phase is leveraged to stabilize Ni-rich cathodes. Density functional theory reveals a reduction in Ni 3d-O 2p hybridization in NiO compared to LiNiO₂ (LNO), suggesting its potential as a protective layer. Guided by theory, variable temperature X-ray diffraction is used to identify optimal conditions for introducing oxygen vacancies on the surface of LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) particles, which triggers a phase transformation from layered to rock-salt NiO on the surface, creating a core-shell structure as evidenced by X-ray photoelectron spectroscopy and scanning transmission electron microscopy (STEM). Electrochemical methods such as constant-current long-term cycling, cyclic voltammetry, and electrochemical impedance spectroscopy reveal improved capacity, higher Li⁺ diffusivity, and lower resistance during cycling. X-ray absorption spectroscopy confirms that the bulk-averaged oxidation state remains unchanged after modification, and STEM imaging confirm reduced structural heterogeneity. By reframing surface NiO as a controllable design principle, a materials-intrinsic, scalable route to extend the durability of Ni-rich cathodes is offered.