In this study, the projected density of states (PDOS) of the stable normal-spinel structure and stable Mg/Co mixed-cation spinel structure of Mg 1+y Co 2−x Mn x O 4 (x = 0, 0.5; y = 0, 0.5, 1) in the pristine and discharged states are obtained using first-principles calculations. The spin state and the valence state of the transition metals are investigated. The overlaps of the d orbitals of the transition metals and the p orbitals of oxygen are large, and the covalency between the transition

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... tal and oxygen is strong in the pristine MgCo 2 O 4 and MgCo 1.5 Mn 0.5 O 4 . The M-O 6 (M = Co, Mn) octahedra become stable as a host structure. From the PDOS spectra, Co atoms are in the trivalent low-spin state in pristine MgCo 2 O 4 and MgCo 1.5 Mn 0.5 O 4 and Mn atoms are nearly tetravalent in pristine MgCo 1.5 Mn 0.5 O 4 . In the discharge process, the overlap of the d orbitals of the transition metals and the p orbitals of oxygen becomes narrow and the valence of the transition metals decreases with increasing Mg insertion. The results of the first-principles calculations are consistent with those of X-ray absorption near edge structure spectra. laptops and smartphones. In the future, the application range of secondary batteries will include large equipment such as electric vehicles, which will greatly reduce the environmental load generated by the transportation sector. However, Li metal is scarce because of its small reserves. Recently, Mg metal, which is earth-abundant, has been attracting attention as a next-generation secondary battery material. Mg secondary batteries are expected to exhibit a greater energy density than Li + -ion batteries. V 2 O 5 , 1 MnO 2 , 2 MoO 3 , 3 polyanionic compounds (olivine-type Mg x MSiO 4 (M = Fe, Mn, Co), 4-6 MgFePO 4 F, 7 NASICON-type Mg x V 2 (PO 4 ) 3 (0¯x¯1.5)), 8 and spinel-structured § ECSJ Active Member Y. Idemoto orcid.org/0000-0003-3832-2139 Electrochemistry