Sulfur in hollow nanofibers overcomes challenges of lithium-ion battery design

Oct 05, 2011 BY SARAH JANE KELLER
A scanning electron microscope photo of hollow carbon nanofiber-encapsulated sulfur tubes, at the heart of a new battery design. Image: Wesley Guangyuan Zheng

(PhysOrg.com) -- Stanford researchers have used nanotechnology to invent a better lithium ion battery cathode.

The design of today's rechargeable lithium ion batteries limits the use of new technologies like and grid-scale energy storage because they do not store enough energy relative to their volume and weight – or, as researchers would say, their energy density is too low.

Solving that problem is largely a matter of finding new materials for the positively and negatively charged electrodes, the cathode and anode.

The research group of battery inventor Yi Cui, an associate professor of materials science and engineering, uses nanotechnology to fabricate electrode materials that greatly improve the electrical storage capacity of lithium ion batteries. In previous research, they reinvented battery anodes by fabricating them with silicon nanowires.

Now, Cui and his students have used sulfur-coated hollow carbon nanofibers and a special electrolyte additive to improve the other end of the rechargeable , the cathode. The results were published online Sept. 14 in the journal Nano Letters.

According to Cui, putting silicon nanowire anodes and sulfur-coated carbon cathodes into one battery is the next generation of battery design.

"I strongly believe that's a promising future choice to make better batteries," Cui said.

 " is one of the materials that can offer a 10-times higher charge storage capacity but with about half the voltage of the existing battery," he said.

 Both the charge capacity and the voltage affect how much energy a battery can deliver. With the sulfur cathode as part of a complete battery, the higher charge capacity makes it possible to build a battery with four to five times the energy storage compared to existing lithium ion battery technology.

Lithium-sulfur batteries have received attention because of the low cost and non-toxicity of sulfur. However, previous generations of lithium sulfur cathodes have not been viable for commercialization because they rapidly fail from repeated charging and recharging.

The new cathode fabrication resolves a number of material issues that, Cui said, "added together represent a really big challenge to get this material to work as a viable battery."

In previous lithium-sulfur cathode designs, sulfur coats onto relatively open carbon structures. This is a problem because it exposes sulfur to the battery's electrolyte solution. When intermediate reaction products called lithium polysulfides come into contact with the electrolyte solution, they reduce the battery's capacity by dissolving into the electrolyte.

As Cui's graduate student, Wesley Guangyuan Zheng, explained, "This can be conflicting because on the one side we don't want a large surface area contacting the sulfur and the electrolyte, and on the other hand we want a large surface area for electrical and ionic conductivities."

The new design solves the conflict with a unique fabrication process that allows sulfur to coat the inside of a hollow carbon nanofiber, but not the outside. This fabrication process relies on a novel use of a commercially available filter technology that is normally applied to water filtration.

The new design also improves battery capacity because it has a nearly closed structure that prevents polysulfides from significantly leaking out into the electrolyte solution. The length of a hollow nanofiber is about 300 times its diameter; the long and narrow channels prevent polysulfides from leaking out.

In addition to the gains made with improved sulfur hollow carbon nanofiber fabrication, Cui's graduate student Yuan Yang included an electrolyte additive that enhances the battery's charge and energy efficiency, known as the coulombic efficiency.

"Without the additive you put 100 electrons into the battery and you get 85 out. With the additive, you get 99 out," Cui said.

"To design the best structure we need both the electrode design and the electrolyte additive and these two combined together can give you a high capacity and high coulombic efficiency," Cui said. "We now have high capacity on both sides of the electrode; that's exciting."

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Vlasev
4.5 / 5 (2) Oct 05, 2011
99% coulombic efficiency!
To me this sounds like:
- minute quantities of heat while charging/discharging
- low capacity loss per charge cycle

Yet, nothing like that was stated in the article. I'm puzzled, and the implications I came up with might be incorrect.
Scottingham
5 / 5 (4) Oct 05, 2011
Battery technology researchers > bankers
that_guy
3.7 / 5 (3) Oct 05, 2011
I think we all get tired of the 'better battery' stories that never seem to pan out, but from what I see, this does sound promising.

If this really hits the ground in the future, it has just enough capacity to become revolutionary. A Nissan leaf that goes 500 Miles on a charge (IE farther than 95% of cars go on a tank of gas), a smartphone that lasts a week on a charge. A dumbphone that lasts a month or more...etc.
pokerdice1
5 / 5 (1) Oct 06, 2011
Well I'm cautiously hopeful, but I would still like to see concrete numbers on the number of recharges you can get from this. The half voltage issue is definately a detractor as that wasn't addressed by the innovation.

Here is my question: is the possible necessity of a transformer to upstep the current a dealbreaker in the electric car business?
Vlasev
5 / 5 (1) Oct 06, 2011
...I would still like to see concrete numbers on the number of recharges you can get from this. The half voltage issue is definately a detractor as that wasn't addressed by the innovation.

Exactly what I was talking about: 99% efficiency should mean only 1% of the energy input dissipates as heat or deterioration processes. However they've mentioned "poly-sulfides leakage" process which decreases the capacity.

Half-voltage should not be an issue - you can get twice the voltage connecting two units consecutively (not sure if i used the correct term)


Here is my question: is the possible necessity of a transformer to upstep the current a dealbreaker in the electric car business?


For what I know transformers are used to up-step/down-step voltage and not current. Both properties can be manipulated trough consecutive and parallel connections between battery units. And I don't mean transformers and other devices are not required to boost efficiency and usability.
jselin
5 / 5 (2) Oct 06, 2011
@Pokerdice1
No need... batteries in series add voltage. You can pick whatever multiple of the cell voltage you want
that_guy
5 / 5 (1) Oct 06, 2011
@Pokerdice1
No need... batteries in series add voltage. You can pick whatever multiple of the cell voltage you want

I was thinking the same thing - aside from how it affects the energy density, I was wondering why voltage was a big deal, because you can always step it up. I didn't comment on that because I was too lazy to look it up and verify it.
pokerdice1
not rated yet Oct 09, 2011
lol careless of me, yeah I meant upstep the voltage, not current.

Another question I have: How long to charge at a recharge station? So, it seems we addressed issues of capacity and expense to a large degree (still I wonder what this unnamed efficiency boosting additive is composed of and its cost).

It seems to me the barrier to common feasibility is the recharge time. Could this issue be addressed by combining the electrode geometries mentioned in...

http://www.physor...ery.html

Spheres in the cathode, for fast cascades, mentioned in the above article would not be amenable to the melting techniques discussed in the article. The melting characteristics are vastly different, yet we have the cathode structure for a fast recharge.