First-Principles Studies of Structural Evolutions in Cathode Materials LiMO₂ (M=Co, Mn, Ni)

We explore the structural evolutions of stoichiometric LiMO₂ using the first-principles calculations combined with the cluster expansion method. We automatically obtain the ground state structures of the stoichiometric LiMO₂ by just considering the cation orderings in the quasi rock-salt structures and the following structural relaxations due to both the atomic size mismatches and the Jahn-Teller distortions. We point out that, on the one hand, the cation orderings are mainly determined by the nearest, the second nearest, and the third nearest cation interactions and can be obtained from the 'phase diagram' we have built using the relative strengths of effective cluster interaction (ECI). On the other hand, the structural relaxations are dominated by the crystal field splitting (CFS) energies, i.e., structures with larger CFS energies are more stable. By calculating the ECIs and CFS energies for various structures of LiMO₂, we clearly show how ECI and CFS play roles in determining the structural evolution mechanism of these systems.