Improved Non-Grignard Electrolyte Based on Magnesium Borate Trichloride for Rechargeable Magnesium Batteries

Kazuhiko Sato; Goro Mori; Takahiro Kiyosu; Toyonari Yaji; Koji Nakanishi; Toshiaki Ohta; Kuniaki Okamoto; Yuki Orikasa

doi:10.1038/s41598-020-64085-2

Improved Non-Grignard Electrolyte Based on Magnesium Borate Trichloride for Rechargeable Magnesium Batteries

Sci Rep. 2020 Apr 30;10(1):7362. doi: 10.1038/s41598-020-64085-2.

Authors

Kazuhiko Sato¹, Goro Mori¹, Takahiro Kiyosu¹, Toyonari Yaji², Koji Nakanishi^{2

3}, Toshiaki Ohta², Kuniaki Okamoto¹, Yuki Orikasa⁴

Affiliations

¹ FUJIFILM Wako Pure Chemical Corporation, 1633 Matoba, Kawagoe, Saitama, 350-1101, Japan.
² SR Center, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga, 525-8577, Japan.
³ Laboratory of Advanced Science and Technology for Industry, University of Hyogo, 3-1-2 Koto, Kamigori-cho, Ako-gun, Hyogo, 678-1205, Japan.
⁴ Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga, 525-8577, Japan. orikasa@fc.ritsumei.ac.jp.

Abstract

The high anodic stability of electrolytes for rechargeable magnesium batteries enables the use of new positive electrodes, which can contribute to an increase in energy density. In this study, novel Ph₃COMgCl-, Ph₃SiOMgCl-, and B(OMgCl)₃-based electrolytes were prepared with AlCl₃ in triglyme. The Ph₃COMgCl-based electrolyte showed anodic stability over 3.0 V vs. Mg but was chemically unstable, whereas the Ph₃SiOMgCl-based electrolyte was chemically stable but featured lower anodic stability than the Ph₃COMgCl-based electrolyte. Advantageously, the B(OMgCl)₃-based electrolyte showed both anodic stability over 3.0 V vs. Mg (possibly due to the Lewis acidic nature of B in B(OMgCl)₃) and chemical stability (possibly due to the hard acid character of B(OMgCl)₃). B(OMgCl)₃, which was prepared by reacting boric acid with a Grignard reagent, was characterized by nuclear magnetic resonance (NMR) spectroscopy, Fourier-transform infrared spectroscopy (FTIR), and X-ray absorption spectroscopy (XAS). The above analyses showed that B(OMgCl)₃ has a complex structure featuring coordinated tetrahydrofuran molecules. ²⁷Al NMR spectroscopy and Al K-edge XAS showed that when B(OMgCl)₃ was present in the electrolyte, AlCl₃ and AlCl₂⁺ species were converted to AlCl₄^-. Mg K-edge XAS showed that the Mg species in B(OMgCl)₃-based electrolytes are electrochemically positive. As a rechargeable magnesium battery, the full cell using the B(OMgCl)₃-based electrolyte and a Mo₆S₈ Chevrel phase cathode showed stable charge-discharge cycles. Thus, B(OMgCl)₃-based electrolytes, the anodic stability of which can be increased to ~3 V by the use of appropriate battery materials, are well suited for the development of practical Mg battery cathodes.