Oxygen Activities Governing Structural Reversibility in Industrial Ni-Rich Layered Cathodes

The chemical reactions and phase transitions at high voltages determine the electrochemical properties of high voltage layered cathodes such as Ni-rich rhombohedral materials. Here, we performed a comprehensive and comparative study of the cationic and anionic redox reactions, as well as the structural evolution of a series of industrial Ni-rich layered cathode materials with and without Al doping, which are being utilized in the cells made by LG Energy Solutions Co.. We combined the results from X-ray spectroscopy, operando electrochemical mass spectrometry, and neutron diffraction with electrochemical properties, and revealed the different oxygen activities associated with structural and electrochemical degradations. We show that Al doping suppresses the irreversible oxygen release thereby enhancing the reversible lattice oxygen redox resulting from the interplay between static (doped Al) and dynamic disorders (reversible oxygen redox). With this modulated oxygen activity, the Ni-rich cathode's notorious H2-H3 structural phase transition becomes highly reversible. Our findings disentangle the different oxygen activities during high-voltage cycling and clarify the role of dopants in the Ni-rich layered cathodes in terms of structural and electrochemical stability finally making all the cell makers get back to the fundamental investigation regarding whether high-Ni NCM chemistry (NCM811 or NCM 91/2 1/2) is substantially beneficial compared to its mid-Ni homologues (NCM622).