On the Origin of Reversible and Irreversible Reactions in LiNi$_{x}$Co$_{(1-x)/2}$Mn$_{(1-x)/2}$O$_{2}$

Bond formation and breakage is crucial upon energy storage in lithium transition metal oxides (LiMeO$_{2}$, Me = Ni, Co, Mn), i.e., the conventional cathode materials in Li ion batteries. Near-edge X-ray absorption finestructure spectroscopy (NEXAFS) of the Me L and O K edge performed upon the first discharge of LiNi$_{x}$Co$_{(1-x)/2}$Mn$_{(1-x)/2}$O$_{2}$ (x = 0.33: NCM111, x = 0.6: NCM622, x = 0.8: NCM811) in combination with charge transfer multiplet (CTM) calculations provide unambiguous evidence that redox reactions in NCMs proceed via a reversible oxidation of Ni associated with the formation of covalent bonds to O neighbors, and not, as widely assumed, via pure cationic or more recently discussed, pure anionic redox processes. Correlating these electronic changes with crystallographic data using operando synchrotron X-ray powder diffraction (SXPD) shows that the amount of ionic Ni limits the reversible capacity— at states of charge where all ionic Ni is oxidized (above 155 mAh g$^{-1}$), the lattice parameters collapse, and irreversible reactions are observed. Yet the covalence of the Ni–O bonds also triggers the electronic structure and thus the operation potential of the cathodes.