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-O6 octahedra were more stable than the Co-O6 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 MgCo1.5Mn0.5O4 was more easily inserted and moved than in MgCo2O4 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.

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The anion-water bonds and hydrogen bonds between water molecules in X(-)(H(2)O)(n) (X = F and Cl, n = 3-7) clusters are analyzed by evaluating the charge-transfer (CT) and dispersion terms for every pair of ions and molecules with the perturbation theory based on the locally projected molecular orbitals. In particular, the relative stabilities and the bond strengths in all 11 distinct cubic X(-)(H(2)O)(7) isomers are analyzed by classifying the ligand water (L) with the numbers of the donating (n) and accepting (m) OHs as LD(n)A(m). The number of LD(0)A(2) waters determines the relative stability. It is demonstrated that the strengths of the anion-ligand bonds are strongly influenced by two other hydrogen bonds of the water molecules adjacent to the ligand. When the model theory of Mulliken's charge-transfer interaction is applied to the anion-ligand and water-water hydrogen bonds, the dependence of the bond strengths on the chains of the hydrogen bonds is explained.

MgCo 2−x Mn x O 4 (x = 0.1, 0.2, 0.4) was prepared using an inverse co-precipitation method. The primary product was determined to have a spinel structure (space group F d 3m) based on powder X-ray diffraction data. A Rietveld analysis of synchrotron X-ray diffraction data showed that Mg, Co and Mn in this material were distributed in a disordered manner, meaning that cation mixing had occurred. Charge-discharge testing using MgCo 2−x Mn x O 4 /AZ31 cells with Ag reference electrodes demonstrated a discharge capacity of 80 mAhg −1 and a high coulombic efficiency below 60°C, with cutoff voltages in the range of 0.345 to −1.155 V vs. Ag/Ag + (3.5 to 2.0 V vs. Mg/Mg 2+). The improved cycling performance of this material is ascribed to the replacement of a portion of the Co atoms with Mn. The stability of the crystal structure was investigated based on first-principles calculations and the results showed that a model in which Mn occupied only the 16d sites was the most stable. The ordered Mg/Co/Mn structure of this material would be expected to facilitate the diffusion of Mg 2+ ions throughout the cathode material in a magnesium secondary battery.

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