Whether sulfur is a by-product or a waste product of oil refinement and coal combustion depends on how you slice it. Certainly, much of that sulfur can be put to use producing sulfuric acid, fertilizer and other chemicals, but some is left to accumulate on stockpiles which are expensive to maintain (due to the need to neutralize acidic run-off). Researchers at the University of Arizona think more of that sulfur could one day be put to use thanks to a new chemical process that uses sulfur to make polymers. The new material could lead to a new generation of lighter, more efficient lithium-sulfur batteries, the researchers claim.

Co-author Jared Griebel calculates that every 19 gallons of gasoline produced leaves half a pound (230 grams) of sulfur as a waste product. Of the 60 million tons of sulfur produced ever year, 7 million tons are surplus to requirements.

A sulfur stockpile at the processing facilities of Syncrude in Alberta, Canada (Photo: David Dodge/The Pembina Institute)

Conscious of all that sulfur was sitting idly by, lead researcher, chemist Jeffrey Pyun, set himself the problem of investigating how "the garbage of transportation" (as he terms the waste sulfur) might be used to make a new kind of lithium-sulfur (Li-S) battery. The new approach was to use liquid sulfur to make a moldable polymer, a use for which sulfur isn't ordinarily suited due to its unwillingness to form polymer chains.

The researchers identified more than 20 chemicals deemed most likely to polymerize sulfur, and set about testing them one by one, by dissolving them in liquid sulfur. The first one they tried, divinylic styrenic comonomers, happened to be the best of the lot. Only a relatively small amount of this chemical additive need be used, leading to the researchers naming their process inverse vulcanization (as vulcanization involves adding a small amount of sulfur to rubber for durability). Imprint lithography was used to create films of the material.

The researchers report that the sulfur polymer exhibited "comparable electrochemical properties to elemental sulfur," with a specific capacity of 823 mAh/g after 100 charge cycles, but with the additional benefits of being readily soluble and meltable (unlike elemental sulfur). The specific capacity recorded is thought to be the highest yet for a polymeric material.

Though it's not an immediate silver bullet in boosting the performance of Li-S batteries, it's hoped that the development could lead to a new class of battery applications in the future. The researchers suggest the new material might lend itself to optical applications, too.

The team's research was published in Nature Chemistry on April 14.

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