Abstract: The layered Ni-rich cathode materials are considered as the most promising cathode materials for Li-ion batteries due to their high reversible capacity, low self-discharge performance and low cost. However, they have some disadvantages, such as the unstable material structure, capacity decay and poor safety, hindered their practical application. The Ni-rich cathode materials with nickel content over 80% are easy to react with moisture and CO₂ in the air, generating residual Li compounds such as Li₂CO₃, LiHCO₃ and LiOH on the surface of materials. The presence of residual Li not only leads to structure instability and electrochemical performance degradation, but also causes battery safety problems. In this paper, the mechanism of the formation of residual Li and its hazards are reviewed. Then, the effect of factors (e.g. washing temperature, time, drying temperature, etc.) during water washing on the material property are discussed, and the mechanisms of structural deterioration and capacity degradation induced by water washing are elaborated. Lastly, other methods for removing residual Li compounds are introduced, especially the non-washing surface coating method, which shows great application potential in removing the influence of residual Li compounds.