Study of Interfacial Degradation Reactions via Operando Soft X-ray Absorption Spectroscopy on Ni-Rich NMC811 Cathodes in Lithium-Ion Batteries

Abstract

Ni-rich layered NMCs are a very promising class of cathode materials for commercial lithium-ion batteries with high energy and power densities, finding application in many portable devices and electric vehicles. However, upon extended cycling and deep-charging of the cathodes, they undergo irreversible capacity losses, severely limiting battery lifetime. Understanding interfacial and bulk processes behind cathode degradation can provide insight into necessary material enhancements. In this work, we performed operando and ex situ near-edge X-ray absorption fine structure (NEXAFS) of oxygen, fluorine, cobalt, and nickel in NMC811∥Li batteries as a function of cathode lithiation. We find that one of the driving forces for cathode degradation is the oxygen 2p─nickel 3d charge transfer at high states-of-charge (SoC), due to the increased Ni–O covalency beyond Ni3+. In the bulk, this leads to reversible formation of nickel peroxide species, while at the interface, irreversible phase change toward a rocksalt structure occurs. This causes local oxygen reduction into reactive 1O2, concomitant with the reduction of present transition metals, forming an irreversible, redox-inactive surface layer that grows over the course of battery cycling. These results expand the current knowledge of NMC battery degradation and pave the way for designing stable, high capacity cathodes in contemporary lithium-ion batteries.