Sulfone-Based Electrolytes for Magnesium Rechargeable Batteries

Magnesium (Mg) is an attractive candidate for the anode material of a novel high-performance rechargeable battery because of its high energy density, moderate electrochemical activity, chemical stability, as well as the natural abundance. One of the most important issues to realize the Mg rechargeable battery is the development of electrolytes. Aurbach et al. have developed electrolyte solutions in which Mg is deposited and dissolved reversibly, that are formed by reacting R₂Mg or RMgCl Lewis base with XR_3-nCl_n Lewis acid (n=0-3, X=Al, B) in ether solvent. Here the transmetalation between Lewis base and acid is a key step in the formation of the electrochemically active products, and the Lewis acid is the essential factor in improving the electrochemical properties such as reversibility, conductivity, as well as electrochemical windows. As a result, a magnesium organohaloaluminate in tetrahydrofran (THF) with a formula of 0.25 mol/L Mg(AlCl₂EtBu)₂/THF shows nearly 100% columbic efficiency, 2.5V electrochemical window, and room-temperature conductivity in the range of several millisiemens, which are much better than those of conventional Grignard reagents.

After the breakthrough by Aurbach described above, various approaches have been tried for further improvements. Kim et al. enhanced the electrochemical performance of hexamethyldisilazide magnesium chloride (HMDSMgCl) in THF through the addition of aluminum chloride (AlCl₃) together with the crystallization of electrochemical active species [Mg₂(μ-Cl)₃·6THF]⁺. Mohtadi et al. reported the reversible Mg electrolytes of magnesium borohydride Mg(BH₄)₂ in both THF and dimethoxyethane (DME). And recently, Abe et al. have found that magnesium deposition and dissolution can take place in some electrolyte solutions, such as magnesium bromide together with magnesium ethoxide in 2-methyltetrahydrofuran( (Mg(OEt)₂ + MgBr₂)/2-MeTHF), and MgBr₂ in n-butyl methyl ether (MgBr₂/n-BME). The same group has also found that Glymes and magnesium chloride (MgCl₂) with AlCl₃enabled the magnesium deposition and dissolution on platinum electrode with high coulombic efficiency.

Despite the improvements in the electrochemical properties, at least two more breakthroughs are required for the commercialization regarding Mg electrolytes; one is the reduction of chemical reactivity, and the other one is suppression of solvent evaporation. Here we report the development of a novel Mg electrolyte using sulfone as solvents. Although it does not include highly reactive materials such as organic magnesium compounds as well as Lewis acids, the electrolyte shows reversible Mg deposition and dissolution at room temperature. The electrochemical window is larger than 3.0V vs Mg without any corrosion on stainless steel electrodes. As the boiling point of the sulfone is high, we don’t need to care about the solvent evaporation at the operating condition. In addition to the details of our Mg electrolyte, the performance of Mg rechargeable battery whose energy density would be higher than the present Li-ion batteries, will be presented at the conference.