First-principles calculations on Li xNiO 2: phase stability and monoclinic distortion

Abstract

The phase diagram of Li x NiO 2 (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 NiO 2 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, Li 0.4NiO 2, 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 Li 0.75NiO 2 and Li 0.4NiO 2. The coupling between the Li-vacancy ordering and the Jahn–Teller activity of Ni 3+ ions plays a crucial role in the stability of ordered Li x NiO 2 structures and is at the origin of the monoclinic distortion. As a result, the different electrochemical behavior of Li x NiO 2 versus Li x CoO 2 lies in the electronic nature of the involved transition metal cation.