Transition Metal Segregation and Phase Transformations on the Surfaces of Layered Li(Ni1-x-yMnxCoy)O2 (NMC) Cathode Materials for Li-Ion Batteries

Abstract

Nickel-rich layered oxide cathode materials have seen widespread deployment due to their high gravimetric capacity and high average discharge voltage. However, achieving this high capacity requires charging voltages above the potential at which the electrolytes decompose at the cathode surface (>4.5 V vs. Li/Li+), which leads to impedance rise and capacity loss. Furthermore, the reduction of the transition metals and oxygen loss produces a phase transformation that might be detrimental to the electrochemical performance. One strategy to stabilize this interface and improve the cycling of cathode materials is by the use of a surface coating. In this talk, we present recent Density Functional Theory (DFT) efforts aimed at gaining insights on the processes occurring at the cathode surfaces and the effect of alumina coating. It will be shown that Ni and Co possess distinctive roles in the formation of surface segregation layers (SRLs) on Li(Ni1-x-yMnxCoy)O2 NMCs. The results show that Al prefers to be surrounded by Co and Ni cations in the transition metal layers of NMC structures. The existence of Mn in the transition metal layer can therefore hinder the intercalation of Al from the surface to the bulk of the NMC particles. A discussion on the thermodynamics that govern such processes will be given. The authors gratefully acknowledge support from the U. S. Department of Energy (DOE), Vehicle Technologies Office. Computer time allocations at the Argonne's Laboratory Computing Resource Center, is gratefully acknowledged. This research also used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Argonne National Laboratory is operated for DOE Office of Science by UChicago Argonne, LLC, under contract number DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.