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

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 Ph(3)COMgCl-, Ph(3)SiOMgCl-, and B(OMgCl)(3)-based electrolytes were prepared with AlCl(3) in triglyme. The Ph(3)COMgCl-based electrolyte showed anodic stability over 3.0 V vs. Mg but was chemically unstable, whereas the Ph(3)SiOMgCl-based electrolyte was chemically stable but featured lower anodic stability than the Ph(3)COMgCl-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. (27)Al NMR spectroscopy and Al K-edge XAS showed that when B(OMgCl)(3) was present in the electrolyte, AlCl(3) and AlCl(2)(+) species were converted to AlCl(4)(−). 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 Mo(6)S(8) 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.