Discovery of new ground state structures for Li4Mn2O5 and V2O5 from first principles

Abstract

Computational tools are often useful in resolving the crystal structure of newly discovered materials as well as in predicting existence of the undiscovered materials. The current study illustrates the application of a well-known method of cluster expansion from alloy thermodynamics to resolve the crystal structure of a novel promising cathode compound for Li-ion batteries, Li4Mn2O5. We systematically narrow down the possible space of site orderings through the construction of a gradually improved cluster expansion based on the first principles calculation of total energy to obtain the ground state structure of Li4Mn2O5. The new-found ground state is 66 meV/f.u. lower than the previously determined ground state for this material. On the basis of the new-found Li4Mn2O5-structure, we compute the phase stability of the family of the isostructural 3d transition metal oxides and find that the oxides are metastable in both the lithiated and delithiated states. The exception to this general trend is exhibited by vanadium pentoxide, which is found to be robustly stable in both lithiated and delithiated states. The V2O5 phase in the new structure is more stable by over 1 eV/f.u. compared to the well-known ground state polymorph in Pmmn phase. The delithiation of the lithiated phase can provide a plausible synthesis route to this hitherto unknown ground state polymorph of vanadium pentoxide. The current study not only resolves the crystal structure of Li4Mn2O5, but also predicts the ground state polymorph of V2O5 through a computational route.