Search for Spinel Type Cathode Materials of Magnesium Secondary Battery By First-Principles Calculation and the Battery Properties, Crystal and Electronic Structure of Cathode Material

Abstract

INTRODUCTION Currently, the cathode materials of the high-capacity lithium ion battery have been developed. However it is difficult to be tremendous capacity improvement. Therefore divalent magnesium ions have been noted. Due to approximately the same ionic radius of Mg2+ as the Li+, the theoretical capacity of magnesium per volume is higher than lithium, so that magnesium secondary batteries are expected as a next-generation battery. The charge-discharge test of the positive electrode material, such as MgCo2O4, was performed. Although the stable crystal structure and diffusion mechanism of Mg ions is not clear. In this study, the electrochemical evaluation and the crystal structure analysis of the MgCo2O4 and Li2-xCo2O4 were performed and the crystal structure determined by Rietveld analysis. Moreover, the first-principles calculation for MgCo2O4 evaluated the electronic structure especially in the M-O bond. The purpose of this study was to resolve the relationship between the phase stability and Mg/Co mixing in the crystal structure. EXPERIMENTAL MgCo2O4(MCO) was synthesized by reverse co-precipitation method2). Li2Co2O4 was synthesized by solid-state reaction which was chemically delithiated by using the Na2S2O8. The resulting materials were identified by XRD and determined by ICP for the composition of the metals. The charge/discharge test (HS cells : 3.5~0 V vs. Mg/Mg2+; three electrode cells : 0.345 - -0.855V vs. Ag/Ag+, the negative electrode metal Mg or Mg alloy (AZ31), reference electrode : Ag, separator : polypropylene or glass fiber filter, electrolyte : 0.5M-Mg(N(SO2CF3)2)2/CH3CN or Triglyme). These samples were performed by synchrotron X-ray diffraction (BL02B2, SPring-8). It was also investigated for the valence state of Co by XAFS (BL01B1, SPring-8). In addition, the first-principles calculations were performed for the three types of structure models that extends the unit cell of the MCO(VASP-code in 2 × 2 × 1 cell) and were evaluated for binding and the structural stability. RESULTS AND DISCUSSION The XRD for MCO and Li2Co2O4 showed that all of the diffraction peaks were attributed to the spinel structure (S.G.Fd-3m). The Rietveld analysis for the synchrotron X-ray diffraction data showed that Mg and Co in MCO were almost disorderly distributed and cation mixing. Chemically delithitated Li2Co2O4 sample, i.e. Li2-xCo2O4, was synthesized. From the results of charge-discharge test of MCO and Li2-xCo2O4 (counter Mg), the overvoltage of Li2-xCo2O4 was lower than that of MCO, since diffusion path of Mg could be secured by the regular arrangement of Mg/Co. As a result of the charge / discharge test of MCO using three-electrodes cell, we found a plateau of about 2.6 V (Mg/Mg2+) at the discharge (Fig.1). We also investigated the phase stability, Mg/Co mixing, and electronic structure of MCO by first-principles calculations. It was found that the model where the Mg occupied at only 8a site and Co occupied 16d site was stable arrangement. The electron densities between Co-O for all models exhibited higher than those of Mg-O and it means high covalency of Co-O. Therefore, the Mg of the 8a site became weak bond to oxygen to deintercalate. By Mg/Co mixing, the diffusion path of Mg would be inhibited by strong covalent bonds of Co-O. Then, MCO showed larger overvoltage than less mixing of Li2-xCo2O4. It was performed by help of ALCA-SPRING and shows thanks to the members concerned. References 1) N. Kamioka, T. Ichitubo, T. Uda, S. Imashuku, Y. Taninuchi and E. Matsubara, Materials Transactions, 49, 4, pp.824-828 (2008). 2) S.Yagi et al., Jpn. J. Appl. Phys., 52, 025501(2013). Figure 1