Surface phase diagram and stability of (001) and (111) LiMn2O4 spinel oxides

The (001) and (111) surface structures of the LiMn2O4 (LMO) spinel are examined using density functional theory (DFT) calculations within the generalized gradient approximation (GGA) + U approach. In order to clarify discrepancies in the literature for previous DFT calculations of these surfaces, we first carefully study the effects of surface termination/reconstruction, slab construction, and relaxation schemes, as well as magnetic ordering and U values for Mn on the calculated surface energies of LMO. We explain these discrepancies and show that the relaxation scheme and surface reconstruction play the key role in determining the relative stability of (001) and (111) surfaces. We have further analyzed the thermodynamic stability of LMO surfaces as a function of oxygen and lithium chemical potentials. We have found that the ratio of (001) to (111) surface energies is similar to 1.09 up to the oxygen chemical potential corresponding to similar to 800 K. This ratio favors the formation of truncated-octahedron shaped LMO particles dominated by Li-terminated reconstructed (111) facets with no surface Mn, which can help suppress Mn dissolution. Higher temperatures or closed systems (with no exchange of material with surroundings) favor the formation of (001) dominated particles. The observation of a wide spectrum of polyhedral shapes between (001)- and (111)-dominated LMO particles in experiments can be explained by the narrow range of surface energies and their sensitivity to synthesis conditions.