Investigation of Stable Structures and Electronic States of Spinel-Structured MgCo_{2- z}Ni_0.5MnAl_zO₄(Z = 0, 0.3) as Cathode Materials for Magnesium Rechargeable Batteries Using First-Principles Calculations

The stable structures of pristine spinel MgCo2-zNi0.5MnAlzO4 (z = 0, 0.3) are elucidated by first-principles calculations. Electron-density and projected density of states (PDOS) calculations are performed for the z = 0 and 0.3 compounds. The calculated stable structures corresponding to z = 0 and 0.3 are well fitted by pair distribution function (PDF) analysis with the G(r) obtained by synchrotron X-ray total scattering measurements. The PDF analysis reveals that the calculated stable structures of z = 0 and 0.3 agree with the experimental results. The average electron density between the metal on the 8a site and oxygen in the z = 0.3 compound is lower than that in the z = 0 compound because of the reduced cation mixing of Mg/Co in the z = 0.3 compound, indicating weaker bonding. The average electron density between the metal on the 16d site and oxygen in the z = 0.3 compound is higher than that in the z = 0 compound, indicating stronger covalent bonding. The Al-O bonding at the 16d site is more ionic than the Mg-O bonding, and the electrons of oxygen bound to Al are localized, indicating that the electrons are not shared with the surrounding atoms and that covalent bonding is weaker. From our investigation, it could be predicted that the charge and discharge capacities improved by substitution of the Al atom instead of part of the Co atom.