Operando Monitoring of Interfacial Chemistries and Potentials on Ni-Rich NCM811 Using Ambient Pressure X-ray Photoelectron Spectroscopy

Abstract

Ni-rich layered transition metal (TM) oxides, such as Li1+x[Ni0.8Co0.1Mn0.1]1–xO2 (NCM811), are widely used as state-of-the-art cathode active materials (CAMs) in lithium-ion batteries (LIBs). At high Ni-contents, however, these materials suffer from surface instability, oxygen release, and structural degradation upon prolonged cycling. In this work, operando ambient pressure X-ray photoelectron spectroscopy (AP-XPS) was used to follow the real-time surface chemistry and local potentials of NCM811 during the first charge and discharge, with a focus on processes at the CAM/electrolyte interface. To interpret the operando O 1s spectra, a peak-fitting model was developed that allows oxygen-related surface changes to be followed during cycling. Consistent with previous reports, oxygen evolution is detected by online electrochemical mass spectrometry (OEMS) at around 80% state-of-charge (SOC), and is accompanied by changes in the organic electrolyte components and the formation of an oxygen-depleted metal oxide surface layer. Some oxidation-related changes of the electrolyte are partially reversible upon discharge, whereas the oxygen-depleted metal oxide layer persists. By combining operando AP-XPS with OEMS measurements, this work provides a surface-sensitive, operando description of oxygen-release-related interfacial processes and their potential dependence. These results provide a clearer picture of how oxygen-release-driven degradation pathways affect the surface chemistry of Ni-rich CAMs and help identify the processes that need to be controlled to improve surface stability and electrochemical performance in future LIBs.