Insights on the Stabilization of Nickel-Rich Cathode Surfaces: Evidence of Inherent Instabilities in the Presence of Conformal Coatings

Ni-rich LiNi1–x–yMnxCoyO2 (NMC) cathode materials are being aggressively developed for high-voltage applications such as electric vehicles. These materials are desirable because of their high volumetric energy and power densities. However, degradation phenomena related to surface instabilities of highly charged Ni-rich cathodes have been, and remain, a major concern with respect to cycle life and safety of Li-ion cells. Although many reports exist on the properties and behavior of such materials, sufficient advances have yet to be made with respect to the stabilization of cathode surfaces. Herein, we report on an investigation aimed at probing the inherent stability of electrochemically active Niδ+ species at the surface of NMC-type, layered-oxide cathode particles. A model system is developed that allows for studies of both surface and bulk species using hard X-ray absorption and electron energy loss spectroscopies. Spectroscopy data collected on cathode electrodes, coated with Al2O3 via atomic layer deposition, reveals that the coating does not enhance the ability to attain, or maintain, fully oxidized Ni (Ni4+) in the near-surface regions of charged cathode particles. The results suggest an inherent instability of Niδ+ near the surfaces of cathode particles and imply that the strategy of using Al2O3 as a physical barrier coating is not sufficient to overcome these instabilities in layered nickel-rich oxides. Furthermore, the system developed for this work can be used as a tool to probe, in working cells, the efficacy of other strategies such as electrolytes, additives, and coatings in stabilizing surface Niδ+ species in NMC-type cathodes.