LATTICE-GAS-MODEL APPROACH TO UNDERSTANDING THE STRUCTURES OF LITHIUM TRANSITION-METAL OXIDES LIMO(2)

Many lithium transition-metal oxides (LiMO(2), M = Ti, V, Cr, Mn, Fe, Co, Ni) have structures made up of oxygen atoms occupying a cubic close-packed fee or distorted fee lattice, with cations occupying all octahedral interstices. The cation lattice is therefore also fee and the arrangements of the cations on that lattice can be studied with a lattice-gas model. The LiNiO2, layered LiCoO2, spinel LiCoO2, Li2Ti2O4, and gamma-LiFeO2 structures are predicted for appropriate values of first- (J(1)) and second- (J(2)) neighbor interactions within such a model by analogy with binary-alloy materials having similar structures. We are able to assign allowable ranges for the interactions for each of the oxides above based on their position in the J(1)-J(2)-T phase diagram. A surprising result is that the layered and spinel LiCoO2 structures are equally stable over the same wide range of J(1) and J(2), as predicted by mean-field and Monte Carlo results. Although LiMnO2 is structurally related to the above materials, the LiMnO2 structure is not stable for any choice of J(1), J(2), or T. Further neighbor interactions or anisotropies in the near-neighbor interactions are needed to stabilize the LiMnO2 structure.