Crystal and Electronic Structures of MgCo2-xMnxO4 as Cathode Material for Magnesium Secondary Batteries Using First-Principles Calculations and Quantum Beam Measurements

The stable structure of the spinels MgCo2O4 and MgCo1.5Mn0.5O4, as Mg secondary battery cathode materials, was investigated by first-principles calculations. The calculated stable structures were compared with the crystal structures obtained by quantum beam measurements. The effect on the electronic structure of the substitution of Mn in MgCo2O4 was examined. Pair distribution function fitting of the normal spinel of MgCo1.5Mn0.5O4 gave a better agreement with experiments than that of MgCo2O4. It was found that Mg/Co cation mixing decreased by the substitution of Mn, as found for the Rietveld analysis of the synchrotron X-ray diffraction. From electron density analysis, it was expected that the Mn-O-6 octahedra were more stable than the Co-O-6 octahedra because Mn is more attracted to an O atom than a Co atom, that is, the Mn-O bond was stronger than the Co-O bond. The Mg in MgCo(1.5)Mn(0.5)O(4)was more easily inserted and moved than in MgCo(2)O(4 )because the Mg-O bonds near Mn became weak. This fact is consistent with the fact that the first discharge capacity and cycling performance of MgCo1.5Mn0.5O4 were improved over those of MgCo2O4 in charge and discharge tests.