A comprehensive review of foreign-ion doping and recent achievements for nickel-rich cathode materials

Ni-rich layered oxides have the advantages of high energy density, long cycle life and environmental friendliness, which are considered among the most promising cathode materials for high energy density lithium-ion batteries (LIBs). However, problems such as humidity sensitivity, rapid capacity decay and thermal runaway greatly limit their potential applications. While it is widely accepted that the foreign-ion doping is an effective strategy to enhance cathode electrochemical properties, a comprehensive understanding of its functioning mechanisms and rational doping strategies for next generation Ni-rich cathodes are still lacking. In this review, the issues and challenges of Ni-rich layered oxides are thoroughly discussed. In addition, different mechanisms of elemental doping are systematically analyzed, including the pillar effect, the inhibition of Li/Ni mixing, the formation of surface coating, the change of particle morphology, the suppression of oxygen release and the generation of microcracks. Moreover, the characteristics and features of doping elements in different valence states are compared in detail. We also share our perspectives on key doping principles and future cathode designs. This review offers an exhaustive summary of the cathode doping strategies from doping mechanisms to valence states of doping elements, from historical experience to future design, and provides a fundamental understanding of doping modification for Ni-rich materials.