Operando Monitoring of Early Ni-mediated Surface Reconstruction in Layered Lithiated Ni-Co-Mn Oxides

Nickel-rich layered lithiated Ni-Co-Mn oxides (NCMs) are emerging as the most promising candidates for next-generation Li-ion battery cathodes. Progress, however, is hindered by an incomplete understanding of processes that lead to performance-limiting impedance growth and reduced cycling stability. These processes typically involve surface reconstruction and O-2 release at the, cathode surface, both of which are difficult to monitor in the working cell. We demonstrate that online electrochemical mass spectrometry can be used to measure the gas release from NCMs of varying Ni content at practically relevant potentials and under operando electrochemical conditions. We find that for cathode potentials up to 4.3 V (vs Li+/Li) there is virtually no trade-off between Ni-mediated specific-charge enhancement and parasitic surface reactions. However, at potentials greater than 4.3 V, surface-reconstruction processes giving rise to substantial CO2 and O-2 release occur, implying that surface reconstructed layers a few nanometers thick may form already after the first charge. Ni content and the Ni/Co ratio are found to govern the onset, rate, and extent of these surface-reconstruction processes. These results provide novel insights into the role of Ni in governing the surface stability and performance of Li-ion layered oxides.