Synthesis, Crystal Structure and Electrochemical Properties of Rock-Salt Type (Mg,Ni,Co)O2 As Cathode Materials for Mg Secondary Battery

Abstract

INTRODUCTION Recently, research has been conducted on high-performance next-generation batteries to replace secondary batteries in practical use because of the limitations of their capacity and safety. One of those currently studied is a secondary battery that uses divalent Mg2+ as a movable ion. The Mg secondary battery has been expected to be a realization of high energy density storage battery. However, the problem to be solved is slow diffusion of Mg2+ within the crystal structure since strong electrostatic interaction between Mg2+ and O2- ions. Thus, the practical application of magnesium secondary battery has not yet. Ideally, layered rock-salt type (MgNiyM1-y)O2 was expected to be synthesized as a cathode electrode material because layered oxides LiNi0.8Co0.2O2 and LiNi1/3Co1/3Mn1/3O2 have already been put to practical use as the positive electrode material of lithium ion battery. Since high electrochemical characteristics of Ni, Co and Mn showed, we have attempted to synthesized the new rock-salt MgxNiyCo1-yO2, to analyze the crystal structure and to evaluate electrochemical characteristics. EXPERIMENTAL The samples were synthesized by the reverse co-precipitation1). After mixing the metal nitrate solution at a predetermined ratio, was added dropwise to aqueous sodium hydrogen carbonate solution, the precipitate was filtered, washed, dried, and calcined. The samples were characterized by XRD and ICP measurements. Particle morphology, particle size and lattice image were observed by SEM and TEM. These samples were performed by synchrotron X-ray diffraction (BL02B2, SPring-8). The data was refined by the Rietveld technique using Rietan-FP program. By maximum entropy method (MEM) using the Dysnomia program, the electron densities were analyzed using synchrotron X-ray diffraction. We also examined the valence state of Ni and Co by XAFS measurement (BL01B1, SPring-8). The electrochemical measurements were subjected to a charge-discharge cycle test (3.5 - 0 V vs. Mg/Mg2 +, 0.345 - -1.955V vs. Ag/Ag +, 4.5 - 1.5V vs. Li/Li+, the negative electrode: AZ31 or metal Li, reference electrode: Ag, electrolytic solution: 1.0 M-Mg(N (SO2CF3)2)2/triglyme or 1M-LiPF6/EC: DMC (1: 2), separator: glass fiber or polypropylene) at 60 ºC or 25 ºC by using the HS cell and 3-electrode flat cell. Some of the samples were ground in a planetary ball-mill, we tried to improve the cathode performance by reducing the grain size. RESULTS AND DISCUSSION From powder X-ray diffractions, obtained samples were found that a product can be attributed to a single phase of the rock-salt structure with space group of Fm-3m. The ICP analysis indicated that Mg composition was increased by increasing the feed Mg amount. As a result of refinement for the site occupancy with Rietveld analysis, it was suggested that the cation defect slightly exists. The TEM images corresponding to the (111) and (200) plane were observed. Moreover, STEM-EDS indicated that Ni and Co are uniformly dissolved in a local region of the product (Fig.1). According to the above, we have succeeded in the synthesis of new rock-salt type MgxNiyCo1-yO2 of the new composition. From the discharge-charge test results, the discharge characteristics vary depending upon the Mg composition and Ni/Co ratio. The XANES spectra showed that the electrode after the initial discharge was somewhat lower valence than the original powder sample. Therefore, it suggested the insertion of Mg into the MgxNiyCo1-yO2 by the electrochemical discharge. It was performed by help of ALCA-SPRING and shows thanks to the members concerned. References (1) S. Yagi et al., Jpn. J. Appl. Phys., 52, 025501 (2013). Figure 1