Lithium-sulfur (Li-S) batteries, despite their high theoretical specific energy, face practical challenges including polysulfide shuttling and low cell-level energy density.

Researchers at the University of Waterloo have now shown that the lightweight superconductor MgB₂ (magnesium diboride)—the average mass/atom of which is comparable with carbon—as a metallic sulfur host fulfills both electron conduction and polysulfide immobilization properties.

In a paper in the journal Joule the teams reports that, by using first-principles calculations, they found that borides are unique in that both B- and Mg-terminated surfaces bond exclusively with the S_x²⁻ anions (not Li⁺), and hence enhance electron transfer to the active S_x²⁻ ions.

The surface-mediated polysulfide redox behavior results in a much higher exchange current in comparison with MgO and carbon. By sandwiching MgB₂ nanoparticles between graphene nanosheets to form a high-surface-area composite structure, they demonstrated sulfur cathodes that achieve stable cycling at a high sulfur loading of 9.3 mg cm⁻².

They conclude that this new avenue toward Li-S cathodes, when coupled with a protected lithium metal anode, may lead to practical implementation of batteries that are lighter, yet robust.

Resources

• Quan Pang, Chun Yuen Kwok, Dipan Kundu, Xiao Liang, Linda F. Nazar (2018) “Lightweight Metallic MgB₂ Mediates Polysulfide Redox and Promises High-Energy-Density Lithium-Sulfur Batteries,” Joule doi: 10.1016/j.joule.2018.09.024