First-principle calculations of stable configurations and electronic structures of pristine and discharged spinel Mg_1.31V_1.67-xNi_xO₄ (x = 0, 0.13) as cathode materials for magnesium secondary batteries

The stable configurations and electronic structures of spinel Mg_1.31V_1.67-xNi_xO₄ (x = 0, 0.13) in pristine and discharged samples were determined using first-principles calculations. Structural relaxation, transition state, and projected density of states calculations were performed using density functional theory. Structure relaxation of pristine and discharged Mg_1.31V_1.67-xNi_xO₄ (x = 0, 0.13) demonstrated that Mg in the 8a site moved to a vacant 16c site during the discharge process. When the Mg insertion was equal to 0.5, the ratio of the structural change from spinel to rock-salt was approximately 0.5. NEB calculations indicated that the energy barrier for Mg diffusion from the 8a site to the 16c site of Mg_1.31V_1.57Ni_0.1O₄ was lower than that of Mg_1.31V_1.67O₄. Electron density analysis was used to analyze the covalency between the metals and oxygen, confirming that the covalencies of the V-O and the Mg_{8a site}-O bonds in Mg_1.31V_1.57Ni_0.13O₄ were respectively stronger and weaker than that of Mg_1.31V_1.67O₄. Based on the overlap of the d-orbital of vanadium and the p-orbital of oxygen in Mg_1.31V_1.67-xNi_xO₄ (x = 0, 0.13), the covalency of vanadium and oxygen was constructed from the projected density of states.