Size and charge effects on the structural stability of LiMO2 (M = transition metal) compounds

The structural stability of ordered rocksalt structures with composition AMO(2) (A = alkali metal; M = transition metal) is investigated as a function of size and charge of the cations. Many of these materials are of interest for their applications as intercalation electrodes in rechargeable batteries. The ionic size and charge of the A and M cations are controlled by varying the cation-oxygen interaction parameters of a Buckingham interaction potential. Our calculations show that the alpha-LiFeO2 structure is stabilized for A and M cations of approximately the same size because of a favourable electrostatic energy. As size differences between the cations are increased, the layered alpha-NaFeO structure is stabilized. The LiMnO2 and low-temperature LiCoO2 structures are found to be stable only in a narrow region of parameter space. These calculations also show that as the charge on the M cation is reduced, the alpha-NaFeO2 structure is stabilized for a wider range of cation sizes. We compare the results of our calculations with experimental data.