3/11/2014
Sulfur Polymers Could Enable Long-Lasting, High-Capacity Batteries | Chemical & Engineering News
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Home > Sulfur Polymers Could Enable Long­Lasting, High­Capacity Batteries
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Web Date: February 28, 2014
Sulfur Polymers Could Enable Long­Lasting, High­
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Materials: Lithium­sulfur battery electrodes made of an inexpensive sulfur copolymer have one of the highest
energy­storage capacities ever reported
By Katherine Bourzac
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Sulfur Polymers Could Enable Long­
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Department: Science & Technology News Channels: Materials SCENE
Keywords: Lithium­sulfur battery, plastic batteries, sulfur copolymer, inverse vulcanization
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Lithium­sulfur batteries promise to store four to five times as much
energy as today’s best lithium­ion batteries. But some major practical
hurdles have stood in the way of their commercialization. Now
researchers demonstrate that a sulfur­based polymer could be the
solution for lightweight, inexpensive batteries that store large amounts
of energy (ACS Macro Lett. 2014, DOI: 10.1021/mz400649w).
Lithium­sulfur batteries aren’t practical because they don’t last.
“Lithium­ion batteries can last 1,000 charge cycles, but lithium­sulfur
batteries tend to fail before they’re charged 100 times,” says Jeffrey
Pyun, a chemist at the University of Arizona. Elemental sulfur in
these batteries’ cathodes reacts with lithium ions in the electrolyte to
eventually form lithium sulfur salts that deposit and build up on the
electrodes. These side reactions deplete the cathode of its sulfur,
decreasing the storage capacity of the battery and causing structural
problems for the electrodes.
Several groups have extended the lifetimes of lithium­sulfur batteries
by trapping the element inside nanomaterials, which prevents it from
dissolving from the cathode. Unfortunately these cathode materials
are expensive, Pyun says.
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He thinks electrodes made from sulfur polymers, like other plastic
products, should be inexpensive to manufacture on a large scale. The
raw material is abundant: 60 million tons of elemental sulfur are
produced each year, mostly during petroleum refining, and there are
few uses for it.
Sulfur Tangle
To make a sulfur copolymer for battery electrodes, chemists first
heat elemental sulfur (top) to 185 ºC, which forces the sulfur to
polymerize (red). They then add 1,3­diisopropenylbenzene
(blue) and continue to heat the mixture to form the copolymer
(bottom).
Credit: ACS Macro Lett.
Then last year, Pyun’s group reported a way to transform this
elemental sulfur into an inexpensive cathode material (Nat. Chem.
2013, DOI: 10.1038/nchem.1624). By heating the sulfur to 185 ºC
and then adding an organic compound, 1,3­diisopropenylbenzene,
the researchers form a copolymer containing strings of sulfur atoms
tangled up with the diisopropenylbenzene. The organic part of the
polymer doesn’t store energy or participate in reactions in the battery.
Rather, Pyun says, it seems to act as a plasticizer that keeps the lithium­sulfur salts in the reactive mix, rather than in intractable
deposits.
In the new study, the team wanted to find the best cathode material recipe and test it in batteries, something they didn’t do in the
2013 paper. They tested 2,032 copolymers, each with different amounts of sulfur, in coin­cell batteries. These batteries resemble
those used in wristwatches and are the standard way researchers test new battery designs.
http://cen.acs.org/articles/92/web/2014/02/Sulfur-Polymers-Enable-Long-Lasting.html
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3/11/2014
Sulfur Polymers Could Enable Long-Lasting, High-Capacity Batteries | Chemical & Engineering News
The best performing copolymer consisted of 90% sulfur by mass. Batteries using this copolymer had an initial storage capacity of
1,225 mAh per gram of material. After 100 charge­discharge cycles, the capacity dropped to 1,005 mAh/g, and after 500 cycles it
fell to about 635 mAh/g. In comparison, a lithium­ion battery typically starts out with a storage capacity of 200 mAh/g but maintains
it for the life of the battery, Pyun says.
To become a commercial product, a battery made with the sulfur polymer will need to have a steady storage capacity throughout
its lifetime and be able to last the 1,000 cycles of today’s batteries. To get there, Pyun is experimenting with other kinds of sulfur
copolymers that may have better properties.
“This shows that polymeric sulfur is a competitive material” for lithium­sulfur battery electrodes, says Jodie L. Lutkenhaus, a
chemical engineer at Texas A&M University. Lutkenhaus, who is also developing polymer battery materials, says that the
polymers show promise in other ways besides high storage capacity. Polymer batteries are likely to be cheaper and lighter, and
they could enable flexible storage devices for portable electronics. They could even do double duty as a structural material—for
example, as an airplane wing or a car door that also stores energy.
hemical & Engineering News
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ISSN 0009­2347
Copyright © 2014 American Chemical Society
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