There is an increasing demand for energy storage devices due to the development of renewable energy technologies and electric vehicles. The Li-S battery has been studied due to its high theoretical energy density and low cost of sulfur. Sulfur is also nontoxic and abundant. The theoretical volumetric capacity of Mg metal is about twice of the Li metal, and it also has widespread natural abundance. The Mg-S battery is therefore a promising alternative to the Li-S battery. Similar to its Li counterpart, the Mg-S battery will generate Mg polysulfide intermediates during the discharging process. However, unlike the Li polysulfides, the properties of Mg polysulfides are not well studied. The Mg-S battery suffers from slow rate capability, poor reversibility, and Mg anode passivation which may be related to the presence of Mg polysulfides. To study the properties of Mg polysulfides, Mg polysulfide solutions are chemically synthesized. It is found that in Mg polysulfide solutions, precipitates will form over time. The composition of the precipitates is analyzed in this research. The cycling performance of Mg polysulfides are also investigated in coin cells using Mg polysulfide solutions as catholyte in conjunction with the carbon matrix at the cathode and the Mg anode. The side reaction between Mg polysulfides and Mg metal could cause passivation of Mg anodes. The relationship between the side reaction and poor reversibility of the Mg-S battery is also studied. Although the Mg polysulfides have much lower solubility than the Li polysulfides in electrolytes, they have a more significant impact on the Mg-S battery cycling behavior. To push the Mg-S battery technology forward, our fundamental understanding of the electrochemical properties of Mg polysulfides needs to be greatly improved, a goal this work seeks to advance.

Systematic study of the role of Mg polysulfides in the Mg-S battery

ACS Spring Meeting 2021

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