Magnesium metal is attractive as an electrode for high energy density batteries because it has a high negative reduction potential, and the highest volumetric capacity of the practical choices from group I and II metals. Furthermore, Mg does not suffer from dendrite formation—a significant safety issue that has dissuaded the commercialization of rechargeable batteries utilizing a lithium metal.

Sulphur is one of the ideal materials to couple Mg in a high energy density system, because it also has a high theoretical capacity (1,671 mAh g⁻¹ or 3,459 mAh cm⁻³).

Unfortunately, to this point, there has been no electrolyte compatible with both magnesium and sulphur. Mg electrolytes reported are nucleophilic, and, therefore, preclude the use of electrophilic cathodes such as sulphur. Consequently, the authors wrote, “the feasibility and performance of a Mg/S battery is completely unknown, because there is no electrolyte compatible with both Mg and S.”

Here we report the performance enhancement of HMDSMgCl [electrolyte], through the addition of a Lewis acid AlCl₃. Crystallization of the electrochemically active species resulted in a dramatic improvement in the potential stability and coulombic efficiency and, furthermore, it is the critical step in synthesizing a non-nucleophilic electrolyte that is chemically compatible with an electrophilic sulphur cathode. Although the dissolution of sulphur and polysulphides plagues the Mg/S system with rapid fade, we demonstrate a proof of concept for the first rechargeable Mg/S battery.

—Kim et al.

The team assembled Mg/S coin cells with a metallic magnesium anode, separator, and a sulphur cathode consisting of elemental sulphur dispersed in carbon black and a polymeric binder to test the feasibility of the new electrolyte. A typical Mg/S coin cell displayed an excellent capacity of 1,200 mAh g⁻¹ (2,484 mAh cm⁻³, based on the mass of sulphur) for the first discharge.

The unique chemical reactions of individual battery electrodes present new challenges to developing the next generation of high-energy density batteries. Nucleophilic, organomagnesium compounds are, ironically, pivotal in synthetic organic chemistry, but detrimental to electrophilic cathodes such as sulphur. By crystallizing the electrochemically active species from a 3:1 mixture of HMDSMgCl:AlCl₃, we mitigated the nucleophilic attack on the sulphur cathode. The increased coulombic efficiency and wider voltage window were electrochemical benefits also achieved through crystallization.

—Kim et al.

A number of technical issues would need to be resolved before the Mg/S system could be considered a candidate for on-board energy storage in a vehicle, including the formation of polysulphide anions which dissolve into the electrolyte solution, as well as the rate capability of the cell.

Resources

• Hee Soo Kim, Timothy S. Arthur, Gary D. Allred, Jaroslav Zajicek, John G. Newman, Alexander E. Rodnyansky, Allen G. Oliver, William C. Boggess & John Muldoon (2011) Structure and compatibility of a magnesium electrolyte with a sulphur cathode. Nature Communications 2, 427 doi: 10.1038/ncomms1435