Functional Polymers for Next-Generation Magnesium-Sulfur Batteries

Current trends in energy systems advances are inclined towards the development fully electric transportation systems, in a bid to mitigate the effects of climate change. Among the high energy density systems developed, magnesium sulfur (Mg−S) battery, with a high theoretical volumetric energy capacity of 3200 Wh/L, stands out. However, the Mg-S battery has an inherent drawback as it suffers from a phenomenon termed; polysulfide shuttle effect. Specifically, this phenomenon involves the formation of magnesium polysulfide, an intermediate species whose redox shuttle effect leads to the formation of a passivating layer, that results in dramatic capacity fading and short lifetimes of the Mg−S cell, precluding the practical use of this battery system.

Presently, successful means of lessening the polysulfide shuttle effect have been demonstrated. Nonetheless, it is imperative that additional means of further mitigating the polysulfide shuttle effect to allow practical use of Mg−S cells, be explored. Furthermore, there has been limited study of correlations between the structure of sulfur cathode polymeric functional layers and polysulfide transport.

Recently, University of Notre Dame scientists: Hunter O. Ford, Laura C. Merrill, Peng He, Sunil P. Upadhyay, and led by Dr. Jennifer L. Schaefer introduced a novel class of polymers capable of impacting and mitigating the polysulfide shuttle effect in Mg−S cells. They showed that the polymers, that were cross-linked ionomeric networks, could effectively repel polysulfides via physical and electrostatic mechanisms. Their work is currently published in the research journal, Macromolecules.

To start with, the researchers synthesized a series of cross-linked ionomer films consisting of varying chain lengths, anion chemistry, and charge:ether oxygen ratio (Ch:EO). Next, the effects of PEG linker length, tethered anion chemistry, and Ch:EO on ion transport and material structure were investigated. Lastly, the aforementioned materials were characterized in terms of structure, conductivity, ability to repel magnesium polysulfides, and performance enhancement of full Mg−S cells employing the ionomers as separators, in a bid to provide a comprehensive picture of structure – composition – property relationships.

The research team observed that an increase in bound charge content (Ch:EO), resulted in not only the electrolyte swelled networks becoming less conductive to cationic species, but also more greatly restricting polysulfide transport. In addition, rejection of the polysulfide species was attributed to two distinct mechanisms: electrostatic repulsion and physical restriction. This way, they were able to confirm that tethered anions could electrostatically repel negatively charged polysulfide anions in certain circumstances. In other scenarios, incorporation of tethered anion resulted in reduced polysulfide crossover due to simple physical restriction.

In summary, the study by Dr. Jennifer L. Schaefer’s research group demonstrated an in-depth assessment of the applicability of cross-linked ionomer networks of varying poly- (ethylene glycol) diacrylate cross-linker chain length, ionic comonomer chemistry, and comonomer ratio, as polysulfide shuttle inhibiting separators in magnesium−sulfur (Mg−S) batteries. All in all, their work provides a stepping stone towards the engineering of better materials for Mg−S and other metal−sulfur batteries.