Air Instability of Ni-Rich Layered Oxides–A Roadblock to Large Scale Application

Benefiting from the excellent lithium ions diffusion kinetics and considerable discharge specific capacity, Ni-rich layered oxides have become the preferred selection cathode active materials (CAMs) for high energy density Li-ion batteries. However, due to the distinctive electronic structure of nickel ions (Ni^2+/3+/4+) and the strict criteria for sintering conditions, the Ni-rich CAMs inherently suffer from notorious deterioration when in contact with the ambient air. This review provides a comprehensive and critical overview of air instability for Ni-rich CAMs, a neglected but critical issue for large-scale application. The fundamental understanding of derivation of air instability and characterizations is first given, followed by a discussion of evolution behaviors and the corresponding negative influence on fabricating properties of electrodes and electrochemical/safety performance of batteries. Afterward, various material modification strategies for improving air storage stability including pre-treatment by doping and coating and post-treatment by gas, wash, and heat are overviewed. Finally, some perspectives for further exploration to address the air instability issue and pave ways toward Ni-rich CAMs’ practical applications are also proposed.