  Energy density limitations of layered oxides with different Ni contents,
i.e., of the conventional cathode materials in Li-ion batteries, are
investigated across the first discharge cycle using advanced spectroscopy and
state-of-the-art diffraction. For the first time unambiguous experimental
evidence is provided, that redox reactions in NCMs proceed via a reversible
oxidation of Ni and a hybridization with O, and not, as widely assumed, via
pure cationic or more recently discussed, pure anionic redox processes. Once
Ni-O hybrid states are formed, the sites cannot be further oxidized. Instead,
irreversible reactions set in which lead to a structural collapse and thus, the
lack of ionic Ni limits the reversible capacity. Moreover, the degree of
hybridization, which varies with the Ni content, triggers the electronic
structure and the operation potential of the cathodes. With an increasing
amount of Ni, the covalent character of the materials increases and the
potential decreases.
