Future batteries: Lithium-sulfur with a graphene wrapper

December 16, 2014, American Institute of Physics
Schematic of the preparation of a 3-D hierarchically structured graphene-sulfur/carbonZIF8-D composite. Credit: K.Xi/Cambridge

What do you get when you wrap a thin sheet of the "wonder material" graphene around a novel multifunctional sulfur electrode that combines an energy storage unit and electron/ion transfer networks? An extremely promising electrode structure design for rechargeable lithium-sulfur batteries.

Lithium-sulfur batteries are of great commercial interest because they boast theoretical specific energy densities considerably greater than those of their already-well-established cousin, lithium ion batteries.

In the journal APL Materials, from AIP Publishing, a team of researchers led by Dr. Vasant Kumar at the University of Cambridge and Professor Renjie Chen at the Beijing Institute of Technology describe their design of a multifunctional sulfur cathode at the nanolevel to address performance-related issues such as low efficiency and capacity degradation.

Metal organic frameworks (MOFs) have attracted plenty of attention recently, thanks to wide-ranging applications in hydrogen storage, carbon dioxide sequestration, catalysis and membranes. And to create their cathode, the team tapped MOF "as a template" to produce a conductive porous carbon cage—in which sulfur acts as the host and each sulfur-carbon nanoparticle acts as energy storage units where electrochemical reactions occur.

"Our carbon scaffold acts as a physical barrier to confine the active materials within its porous structure," explained Kai Xi, a research scientist at Cambridge. "This leads to improved cycling stability and high efficiency." They also discovered that by further wrapping the sulfur-carbon energy storage unit within a thin sheet of flexible speeds the transport of electrons and ions.

What's behind the improved capacity? Fast charge-transfer kinetics are made possible by an interconnected graphene network with high electrical conductivity, according to the team. Their work shows that the composite structure of a porous scaffold with conductive connections is a promising electrode structure design for .

This work provides a "basic, but flexible, approach to both enhance the use of sulfur and improve the cycle stability of batteries," Xi said. "Modification of the unit or its framework by doping or polymer coating could take the performance to a whole new level."

In terms of applications, the novel battery design's unique integration of energy storage with an ion/electron framework has now opened the door for fabrication of high-performance non-topotactic (not involving a structural change to a crystalline solid) reactions-based systems.

What's next for the team? "We'll focus on fabricating hybrid free-standing sulfur cathode systems to achieve high-energy density batteries, which will involve tailoring novel electrolyte components and building lithium 'protection layers' to enhance the electrochemical performance of batteries," noted Xi.

Explore further: New electrolyte for the construction of magnesium-sulfur batteries

More information: "Graphene-wrapped sulfur/metal organic framework (MOF)-derived microporous carbon composite for lithium sulfur batteries," by Renjie Chen, Teng Zhao, Tian Tian, Shuai Cao, Paul R. Coxon, Kai Xi, David Fairen-Jimenez, R. Vasant Kumar and Anthony K. Cheetham. APL Materials , December 16, 2014: scitation.aip.org/content/aip/ … 12/10.1063/1.4901751

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4 / 5 (1) Dec 16, 2014
Lithium-sulfur batteries are of great commercial interest because they boast theoretical specific energy densities considerably greater than those of their already-well-established cousin, lithium ion batteries.

For anyone interested in the ACTUAL numbers they say that they hope to get about twice the density of conventional Lithium ion. I'm not sure that qualifies as "considerably greater", but with batteries EVERY little bit helps ATM.

In the near future, Li-S batteries are poised to attain both high gravimetric and volumetric energy densities beyond the values of 500 Wh kg −1 and 500 Wh l −1, respectively.
3 / 5 (2) Dec 16, 2014
Much ado in the battery business. I like the idea of flow batteries for stationary locations.
not rated yet Dec 16, 2014
I can't wait for graphene to become affordable. That's the key to so many other advances.
not rated yet Dec 16, 2014
cant beat flow for stationary as far as cost/kwh is concerned. cheap cheap and if that process is modified with an additive it will double its output. wow. Im not telling you what that would be. hahahaha hahahaha ahem.
not rated yet Dec 17, 2014

The "considerably greater" came from the first part of the paragraph you quoted.

lithium-sulfur (Li-S) batteries are attracting significant commercial attention owing to their impressive theoretical specific energy densities approaching 2600 Wh kg −1 (by cell weight) which is considerably greater than the well established lithium-ion (Li-ion) batteries at 130–220 Wh kg −1.

I'd say an order of magnitude is considerably greater! However, the line you quoted stated that they expected to actually get "beyond 500 Wh kg -1", so I guess they were comparing theoretical to practical. In other words, apples to oranges. Still 2-3 times greater seems pretty well worth considering!

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