Determining the crystal and electronic structures of the magnesium secondary battery cathode material MgCo2−xMnxO4 using first-principles calculations and a quantum beam during discharge

Abstract

The stable structures of the spinel compounds MgCo2O4 and MgCo1.5Mn0.5O4 following the insertion of Mg atoms into vacant 16c sites during discharge were investigated using first-principles calculations. During this insertion, Mg atoms at 8a sites were found to migrate to other vacant 16c sites, such that the spinel form transitioned to a rock salt structure. The structural change from the standard spinel phase to a rock salt form was minimal in the case of MgCo2O4, since this change required the insertion of numerous Mg atoms. In contrast, a more pronounced structural change from the normal spinel to a rock salt form occurred in the MgCo1.5Mn0.5O4, as this change required fewer Mg atom insertions. The data suggest that the electron density and bond length between Mg atoms at 8a sites and O atoms in MgCo1.5Mn0.5O4 are both reduced compared to that in MgCo2O4. The Mg atoms in MgCo1.5Mn0.5O4 were determined to readily undergo intercalation as a result of the substitution of Mn atoms.