  The present work reviews the charge compensation in Ni based layered oxides
(LiNi1-xMexO2 with x <= 0.2, Me = Co, Mn, space group R-3m) relating
performance parameters to changes in the electronic and crystallographic
structure of the cathode materials. Upon charge and discharge two fundamentally
different redox mechanisms are observed: At low and medium states of charge
(SOCs) charge compensation takes mainly place at oxygen sites while electron
density is shifted from the oxygen lattice to nickel (formation of sigma
bonds). At high SOCs the shift of electron density from the transition metals
to oxygen (formation of pi bonds) enables an additional redox process but also
oxygen release from the transition metal host structure and subsequent
detrimental reactions. Depending on the Ni:Co:Mn content, both processes lead
to characteristic features in the voltage profile of the cathode materials and
performance parameters like the capacity, the cycling stability and the open
cell voltage become a function of the composition.
