Stable Structure and Electronic Structure for MgCo2-XMnxO4 As Cathode Material for Magnesium Secondary Battery in Discharge Process Using First Principle Calculation

Abstract

1. Introduction Magnesium secondary battery materials expected as a higher energy density rather than Li ion secondary batteries because Mg ion is divalent. The spinel MgCo2O4, a potential magnesium secondary battery cathode material, is a redox-species material where the Co atom was expected to exhibit high voltage. The redox reaction of Co3+/Co4+ is MgCo2O4 → Mg2+ + Co2O4 + 2e– … (1) When the Co atom can be reduced to Co2+, the redox reaction of Co2+/Co3+ is MgCo2O4 + Mg2+ + 2e– → Mg2Co2O4 … (2) The theoretical capacity is 260 mAh/g in both redox reactions (1) and (2). Then, the maximum theoretical capacity is expected to be 520 mAh/g when the Mg atom is completely inserted and detached. In our laboratory, the synthesis and the electrochemical test have been performed for the spinel MgCo2O4 and MgCo2-xMnxO4 (x=0~0.6), and charge/discharge process has been investigated using both experimental and calculation method1). When the charge-discharge test of the spinel MgCo2O4 and MgCo1.5Mn0.5O4 was performed, it was found that the first discharge capacity of MgCo1.5Mn0.5O4 became larger than that of MgCo2O4. Recently, it was cleared that the Mg atoms inserted to a vacant 16c site in the spinel MgCo2O4 in discharge process2). The spinel structure changed to the rocksalt structure in discharge process, but the detail mechanism of Mg insertion and diffusion was not clarified. Then, the aim of this study is the investigation of the stable structure and the electronic structure of the spinel MgCo2O4 and MgCo1.5Mn0.5O4 inserted Mg atoms, the mechanism of the insertion of Mg atoms, and the effect of the substitution of Mn atom by the first-principle calculation. 2. Calculation method The stable structure of the spinel MgCo2O4 and MgCo1.5Mn0.5O4 inserted Mg atoms (shown as Mg1+yCo2O4 and Mg1+yCo1.5Mn0.5O4 (y = 0.125~0.75); y is equal to the amount of Mg atoms), were obtained by GGA+U method (VASP code) in discharge process. The cell size of the spinel MgCo2O4 and MgCo1.5Mn0.5O4 were (1×1×1). Structural relaxation is performed by conjugate gradient method. Cut-off energy is 550 eV. U parameter of Co and Mn atom is 6.0 and 5.0 eV, and J parameter of Co and Mn atom is 0.88 eV, respectively. 3. Results and discussion The stable structure of the normal spinel of MgCo2O4 and MgCo1.5Mn0.5O4 inserted Mg atoms on vacant 16c site were investigated using the first-principle calculation. Figure 1 shows the stable structure of the normal spinel of (a) Mg1.125Co2O4 and (b) Mg1.125Co1.5Mn0.5O4 after the relaxation calculation. From Fig.1 (a), all 8a site of Mg atoms slightly move so much in Mg1.125Co2O4. The total energy of Mg1.125Co2O4 is less stable than the pristine MgCo2O4. In the amount of Mg insertion y = 0.25~0.5 of the Mg1+yCo2O4, Mg atom on the 8a site are slightly closed to another vacant 16c site. The structure changes from the spinel structure to the rocksalt structure in y = 0.25~0.5. The completely rocksalt structure was formed at y = 0.615. The Mg atom inserted to the 16c site near Mn atoms on 16d site, shown as Fig.1 (b). In Mg1.125Co1.5Mn0.5O4, the model which has inserted Mg atom far from Mn atoms became unstable. From Fig.1 (b), two Mg atoms on the 8a site near the inserted Mg atom move to other vacant 16c sites. The structural change of Mg1.125Co1.5Mn0.5O4 is larger than that of Mg1.125Co2O4. The total energy of Mg1.125Co1.5Mn0.5O4 is more stable than the pristine MgCo1.5Mn0.5O4. It becomes unstable when Mg atoms on 16c site and 8a site is too close. So, Mg atom on 8a site move to another vacant 16c site. In Mg1.125Co1.5Mn0.5O4, the structure changes from the spinel structure to the rocksalt structure in y = 0.25~0.615 and the completely rocksalt structure was formed at y = 0.75. For the investigation of the translation of Mg atom from the 8a site to the 16c site, the NEB calculation was performed to the Mg1.125Co2O4 and Mg1.125Co1.5Mn0.5O4. It was indicated that The energy profile of Mg1.125Co1.5Mn0.5O4 became more stable than that of Mg1.125Co2O4 from NEB calculation. Then, it was thought that the Mg atoms were easier to insert to the normal spinel of MgCo1.5Mn0.5O4 than that of MgCo2O4. Acknowledgement This work was partly supported by Advanced Low Carbon Technology Research and Development Program (ALCA-SPRING). We are deeply grateful for the cooperation. 1) Y. Mizutani, C. Ishibashi, N. Ishida, N. Kitamura and Y. Idemoto, of the 84th ECSJ Spring Meeting, 2P09 (2017) 2) S. Okamoto, et al., Adv. Sci., 2, 1500072 (2015). Figure 1