First-principles calculations of lithium ordering and phase stability on LixNiO2

The phase diagram of LixNiO2 (0,x,1) is calculated using a combination of first-principles energy methods and Monte Carlo simulations. The energy dependence of the Li-vacancy configurational disorder is parametrized with a cluster expansion. At room temperature ordered LixNiO2 phases appear in the phase diagram at x51/4, 1/3, 2/5, 1/2, and 3/4. The predicted lithium-vacancy ordering at x51/4 and 1/3 are in good agreement with experiments, while for the other phases no detailed experimental evidence has been reported. We predict a previously undetected phase at x52/5 to dominate the phase diagram at low lithium content. The stability of ordered LixNiO2 structures is determined by short-ranged repulsive in-plane Li-Li interactions and long-range attractive interplane Li-Li interactions. These attractive interplane Li-Li interactions are due to the Jahn-Teller activity of Ni ions. As a result, LixNiO2 behaves fundamentally different from LixCoO2 even though their host structures are identical and Co and Ni have similar ionic sizes.