The phase diagram of Li x NiO2 (0<x<1) and the evolution of the monoclinic distortion as a function of the lithium content are calculated using a combination of first-principles energy methods and statistical–mechanics techniques. As a function of the temperature different ordered Li x NiO2 structures appear in the phase diagram at x=0.25, 0.33, 0.4, 0.5 and 0.75. Noteworthy, a new and unsuspected phase, Li0.4NiO2, dominates the low-lithium region of the phase diagram. In agreement with experimental results, maxima in monoclinicity (a m/b m) are predicted to occur near Li0.75NiO2 and Li0.4NiO2. The coupling between the Li-vacancy ordering and the Jahn–Teller activity of Ni3+ ions plays a crucial role in the stability of ordered Li x NiO2 structures and is at the origin of the monoclinic distortion. As a result, the different electrochemical behavior of Li x NiO2 versus Li x CoO2 lies in the electronic nature of the involved transition metal cation.