Determining the crystal and electronic structures of the magnesium secondary battery cathode material MgCo_2−xMn _xO₄ using first-principles calculations and a quantum beam during discharge

Abstract: The stable structures of the spinel compounds MgCo₂O₄ and MgCo_1.5Mn_0.5O₄ 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 MgCo_1.5Mn_0.5O₄, 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 MgCo_1.5Mn_0.5O₄ are both reduced compared to that in MgCo₂O₄. The Mg atoms in MgCo_1.5Mn_0.5O₄ were determined to readily undergo intercalation as a result of the substitution of Mn atoms. Graphic abstract: [Figure not available: see fulltext.]