The stable structures of the spinel compounds MgCo(2)O(4)and MgCo(1.5)Mn(0.5)O(4)following the insertion of Mg atoms into vacant 16csites during discharge were investigated using first-principles calculations. During this insertion, Mg atoms at 8asites were found to migrate to other vacant 16csites, 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 8asites and O atoms in MgCo(1.5)Mn(0.5)O(4)are both reduced compared to that in MgCo2O4. The Mg atoms in MgCo(1.5)Mn(0.5)O(4)were determined to readily undergo intercalation as a result of the substitution of Mn atoms. Graphic abstract