Synthesis of Mg-Rich Mg-M-O(M = Fe, Ni, Mn) Cathode Materials Using Layered Double Hydroxide Andpositive Electrode Characteristics and Average / Local Structure
Authors: Konishi Yoichiro , Kitamura Naoto , Ishida Naoya , Idemoto Yasushi 
Published: December 2020

1. Introduction

The Mg rechargeable battery (MRB) can be regarded as a next-generation secondary battery that replaces the Li-ion battery, since MRB is expected to have a high volume energy density due to divalent charge carrier, abundant resources, and high safety. However, the diffusion of Mg ions in solids is slower than that of Li ions, and thus there have been no reports of excellent positive electrode materials and they have not been put to practical use. As a new electrode material, we have focused on the disordered rocksalt type positive electrode materials, and tried to synthesize the material with various Mg contents by a reverse co-precipitation method 1). However, the obtained materials had lower Mg contents than expected, and it is hard to prepare a Mg-excess material. Therefore, in this study, as a new synthesis process, we used layered double hydroxide (LDH) as a precursor, and pyrolyzed them to synthesize Mg-excess new positive electrode material Mg-MO (M = Fe, Ni, Mn). For the positive electrode material, the electrode characteristics and the crystal structure were investigated in this study.

2. Experimental

The precursor (layered double hydroxide) was synthesized by a coprecipitation method, and the sample was synthesized by pyrolyzing the precursor 2). As the raw materials, MgCl2 · 6H2O, MCl3 · 6H2O (M = Fe, Ni), and MnCl2 · 4H2O were dissolved in ion-exchanged water so that Mg: M became 2: 1, and then the pyrolysis was performed at 500 ° C. The obtained sample was subjected to powder X-ray diffraction measurement to identify the phase and calculate the lattice constant, and the metal composition was analyzed by ICP-AES. A three-electrode cell (manufactured by Toyo System Co., Ltd.) was utilized for a charge / discharge test at an operating temperature of 90 ° C, and the positive electrode characteristics were evaluated. In addition, synchrotron X-ray diffraction measurement (BL19B2, SPring-8) was performed, and the average structure was clarified by Rietveld analysis (RIETAN-FP). Synchrotron X-ray total scattering measurement (BL04B2, SPring-8) were also performed, and the local structure was investigated by reverse Monte Carlo analysis (RMC Profile).

3. Results

As a result of X-ray diffraction measurement, the peaks could be assigned to the spinel structure (Fd-3m) in the Fe system and the rock salt structure (Fm-3m) in the Ni system. Composition analysis by ICP-AES revealed that all the samples prepared by this new process had Mg-rich composition. As a result of the charge-discharge test, a discharge capacity of Mg1.33Ni0.66O2 was only about 4.5 mAh / g although the capacity increased to about 30 mAh / g by partial substitution for Ni. On the other hand, in the Fe-based sample, the capacity was about 60 mAh / g for Mg2FeO4-δ, and excellent cycle characteristics could be confirmed. Moreover, the discharge capacity of Mg2Fe0.66Ni0.33O2 was about 170mAh / g at the 2nd discharge. Reverse Monte Carlo analysis was performed to clarify the cause of the small discharge capacity of the Ni-based samples. As a result, it was found that Ni tends to aggregate in Mg1.33Ni0.66O2 and the domain can be considered not to participate in charge / discharge. On the other hand, similar phenomenon was not observed in Mg1.33Ni0.5Mn0.16O2. From this, it can be concluded that the amount of Mg and/or synthetic process must be reexamined to improve the electrode property.
This work was supported by JST ALCA-SPRING Grant Number JPMJAC1301, Japan.

References

[1] Y. Idemoto, T. Takahashi, N. Ishida, M. Nakayama, N. Kitamura, Inorg. Chem. ,58 (2019) 5664-5670.
[2] B. Hudcova et al., J. Cleaner Production, 171 (2008) 944-953.

Figure 1