First-Principles Study of Stable Local Structures and Mg Insertion/Detachment Mechanism During Charge–Discharge of Spinel Mg(Co, Ni, Mn, Al)2O4 as Cathode Materials of Magnesium Secondary Batteries

The stable structures of spinel MgCo_0.5–zNi_0.5MnAl_zO₄ (z = 0, 0.3) after discharge and charge are determined using first-principles calculations. In particular, at z = 0.3, the geometrical structure changes after discharging and charging are less pronounced than at z = 0, owing to the suppression of Mg/Co cation mixing. Pair distribution function analysis, based on synchrotron X-ray total scattering measurements, shows that the calculated local structure model during discharge is in good agreement with quantum beam results. Molecular dynamics calculations of the diffusion coefficient and Mg ion conductivity at z = 0 and 0.3, using a generic graph neural network force field, show that Mg diffuses more easily at z = 0 at 25 °C, owing to its higher conductivity.