Synthesis, Battery Characteristics and Crystal and Electronic Structure of Cathode Material Spinel Mg(Co, Ni, Mn)2O4 for Mg Secondary Battery
Authors: Hirata Yuki , Ishida Naoya , Kitamura Naoto , Idemoto Yasushi 
Published: December 2020

1.Introduction
A magnesium secondary battery based on two-electron reaction is expected as a next-generation battery. In this study, we focused on Mg(Co, Ni, Mn)2O4 with spinel structure as a new cathode material. In the previous work, since the energy density of the Li-ion battery tended to increase when Ni is substituted for Co in their cathode materials, a Ni-substituted MgCo2O4 system[1] have been studied. It was also clarified that the discharge capacity increased by substituting Mn for Co in the MgCo2O4[2]. In this study, we adopted the advantages of Ni and Mn to the synthesized new cathode materials of MgCo2-x-yNixMnyO4 in which Co were replaced with Ni and Mn. The purpose of this study was to clarify the relationship between the battery characteristics and the crystal structure by using the three-electrode cell and quantum beam, synchrotron X-ray and neutron, analyses.
2.Experimental
All samples were synthesized via the reverse coprecipitation method. The respective metal nitrates were mixed in the secondary distilled water at a predetermined ratio, further made up to 200 mL and added dropwise to the aqueous sodium carbonate solution at 70 °C, the precipitate was dried, pulverized with a ball mill and then calcined at 400°C. The phase of the obtained sample was identified by powder X-ray diffraction and ICP-AES. The charge/discharge test for synthesized sample was performed using a three-electrode cell (Toyo System Co., Ltd.) (90 ℃, Reference electrode: Ag, Negative electrode: AZ31, Electrolyte: 0.3 M [Mg(G4)][TFSA]2/P13TFSA, Separator: glass fiber filter). Crystal structures of the samples were analyzed by Rietveld method for the results obtained by synchrotron X-ray diffraction (BL19B2, SPring-8) and neutron diffraction (BL20, J-PARC). Furthermore, the valence of the transition metals was estimated by XAFS measurement (BL14B2, SPring-8).
3.Results and Discussion
Samples were synthesized in the different metal ratios of Co, Ni and Mn in MgCo2-x-yNixMnyO4. As a result of powder X-ray diffraction, all samples were assigned to a spinel structure (Fd-3m). From the ICP-AES, although most of the composition ratios could be controlled, the Mg content in all samples tended to be slightly less than 1.0. The lattice constant increased with increasing Ni and Mn substitution. This is considered to be due to the divalent Ni on the hexacoordinated site with a larger ionic radius than those of Co and Mn. From the results of charge and discharge tests, it was found that the Ni-substituted sample showed an increase in the discharge potential, and the both Ni- and Mn-substituted samples showed an increase in the discharge capacity. As a result of Rietveld analysis of the Ni substituted samples, it was found that the substituted Ni also occupied at 8a site of the Mg diffusion path. Therefore, it is considered that the Mg diffusion was hindered and the discharge capacity became lower than that of the non-substituted product [1]. The electrochemical tests using [Mg(G4)][TFSA]2/P13TFSA electrolyte with high oxidation resistance showed the increased charge / discharge capacities after 2 cycles. The Rietveld analysis using neutron diffraction patterns clarified cation mixing and refined the mixing amounts of Co, Ni, and Mn into the 8a site. The overall tendency was that Ni was substituted at the 16d site and Mn was substituted at the 8a site. The valences of each transition metals estimated in the charge / discharge process by XAFS, all the transition metals contributed to the charge / discharge because the valences of Co, Ni, and Mn increased or decreased after the initial charge or discharge. The main feature was that Mn showed a large valence change from 4+ to 3+ [2], and it was found that it greatly contributes to the Mg insertion. Although viewpoint of deterioration needed further improvement, it was concluded that MgCo0.8Ni0.4Mn0.8O4 and MgCo0.5Ni0.5MnO4 showed the discharge capacity of 200mAh/g and were the optimum composition in this samples.
This work was supported by JST ALCA-SPRING Grant Number JPMJAC1301, Japan.
References
1) Okamoto, T. Ichitsubo, T. Kawaguchi, Y. Kumagai, F. Oba and S. Yagi, K. Shimokawa, N. Goto, T. Doi, E. Matsubara, Adv. Sci., 2, 1500072 (2015).
2) Y. Idemoto, Y. Mizutani, C. Ishibashi, N. Ishida, N. Kitamura, Electrochemistry, 87, 220 (2019).

Figure 1