First-principles calculations of lithium ordering and phase stability onLixNiO2

The phase diagram of ${\mathrm{Li}}_{x}{\mathrm{NiO}}_{2}$ $(0<x<1)$ 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 ${\mathrm{Li}}_{x}{\mathrm{NiO}}_{2}$ phases appear in the phase diagram at $x=1/4,$ 1/3, 2/5, 1/2, and 3/4. The predicted lithium-vacancy ordering at $x=1/4$ 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 $x=2/5$ to dominate the phase diagram at low lithium content. The stability of ordered ${\mathrm{Li}}_{x}{\mathrm{NiO}}_{2}$ 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 ${\mathrm{Ni}}^{+3}$ ions. As a result, ${\mathrm{Li}}_{x}{\mathrm{NiO}}_{2}$ behaves fundamentally different from ${\mathrm{Li}}_{x}{\mathrm{CoO}}_{2}$ even though their host structures are identical and Co and Ni have similar ionic sizes.