A review of the origin, adverse influence and modification method of residual surface lithium in Ni-rich cathodes

Ni-rich layered oxides LiNixCoyMn1-x-yO2 (x≥0.6), with a high capacity and energy density, are considered to be the primary cathode materials for next-generation lithium-ion batteries (LIBs). However, its high air sensitivity and residual lithium compounds on the surface limit practical applications. Under the influence of water and CO2, Ni-rich layered oxides are prone to the formation of residual lithium compounds, such as LiOH and Li2CO3, on the surface, which is one of the dominant reasons for the phase change and electrochemical degradation. More seriously, residual lithium deteriorates the coating properties of electrode slurry, increases cell polarization during cycling, and produces different gases, leading to the degradation of battery performance and safety. This review briefly introduces the formation mechanism of residual lithium compounds and discusses the negative influence of residual lithium on the electrochemical characteristics of LiNixCoyMn1-x-yO2 (x≥0.6) cathodes. Furthermore, the modification methods, such as washing, secondary calcination and coatings, as well as the design of concentration gradient materials to eliminate the adversarial effects of residual lithium compounds are introduced in detail.