Cation Ordering and Redox Chemistry of Layered Ni-Rich Li_xNi_1–2yCo_yMn_yO₂: An Operando Raman Spectroscopy Study

Although layered transition metal oxides are state-of-the-art cathode active materials for Li-ion batteries, many fundamental aspects of their operation are poorly understood, in particular, how the local lattice structure and transition metal composition influence their electrochemical activity. In this work, the local structure and redox activity of Ni-rich Li_xNi_1–2yCo_yMn_yO₂ (y = 0.1, 0.2, and 0.33, abbreviated as NCM811, NCM622, and NCM111, respectively) Li-ion cathodes are characterized under standard and overcharge operating conditions with a recently developed operando Raman spectroscopy methodology. Supported by DFT phonon calculations and advanced data analysis methods, we demonstrate that the Raman spectra of NCMs entail spectroscopic signatures of cation ordering phenomena, sequential oxidation/reduction of nickel, and participation of bulk lattice oxygen in the charge-compensation process at a low state of lithiation (SOL). Our methodology enables monitoring such processes during cycling and offers the potential for investigating the mechanisms by which certain strategies (i.e., doping, surface coatings, etc.) ameliorate electrochemical performance.