Scientists increase lithium-sulfur battery lifetime by a factor of 10

Feb 25, 2013 by Lisa Zyga feature
(a) TEM image of the sulfur cathode before discharge. The lithium sulfide (dark) is bonded to the inner wall of the hollow nanofiber (transparent). (b) TEM image of the sulfur cathode after full discharge. The lithium sulfide has shrunk away from the carbon wall, resulting in a loss of electrical contact and capacity decay. (c) TEM image of the polymer-modified sulfur cathode before discharge. (d) TEM image of the polymer-modified sulfur cathode after full discharge. The lithium sulfide remains attached to the carbon wall, improving capacity retention. Credit: Guangyuan Zheng, et al. ©2013 American Chemical Society

(Phys.org)—The world of rechargeable batteries is full of trade-offs. While lithium-ion (Li-ion) batteries are currently the most commercially successful, their low energy density doesn't allow for a long driving range. They are also very expensive, often accounting for half the price of electric vehicles. One alternative is lithium-sulfur (Li-S) batteries, which are attractive for their high gravimetric energy density that allows them to store more energy than Li-ion batteries. And although they still use some lithium, the sulfur component allows them to be much cheaper than Li-ion batteries. But one of the biggest drawbacks of Li-S batteries is their short cycle life, which causes them to lose much of their capacity every time they are recharged.

Now a team of researchers led by Yi Cui, a professor of at Stanford University, has developed a Li-S battery that can retain more than 80% of its 1180 mAh/g capacity over 300 cycles, with the potential for similar capacity retention over thousands of cycles. In contrast, most Li-S batteries lose much of their capacity after a few tens of cycles.

To achieve this improvement, the researchers first identified a new mechanism that causes capacity decay in Li-S batteries after cycling. In order for a Li-S battery to successfully recharge, the lithium sulfide in the cathode must be bound to the cathode surface—in this case, the of the hollow carbon nanofiber that encapsulates it. This binding creates a good to allow for charge flow. But the researchers found that, during the discharge process, the lithium sulfide detaches from the carbon, resulting in a loss of electrical contact that prevents the battery from fully recharging.

Before now, it has been very challenging to study the sulfur cathode at the due to the sulfur compound's sensitivity to air and moisture, as well as its tendency to sublime under a vacuum. But the hollow carbon nanofiber structure of the anode—which the researchers developed in a previous study—protects the sulfur, which allowed the researchers to view the using a transmission electron microscope (TEM) without significantly damaging the sample.

After identifying the problem, the researchers set about fixing it by adding polymers to the carbon surface in order to modify the carbon-sulfur interface. The polymers are amphiphilic, meaning they are both hydrophilic (water-loving) and lipophilic (fat-loving), similar to soap. This property gives the polymers anchoring points that allow the lithium sulfides to bind strongly with the carbon surface in order to maintain strong electrical contacts.

As experiments showed, sulfur cathodes containing the amphiphilic polymers had very stable performance, with less than 3% capacity decay over the first 100 cycles, and less than 20% decay for more than 300 cycles.

Although the improvement is a big step forward, the capacity retention still doesn't compare to Li-ion batteries, some of which have lifespans approaching 10,000 cycles. In order to avoid having to replace the battery every few years, require these longer lifespans. But Cui says that Li-S batteries have the potential to close this gap in the foreseeable future.

"Using the amphiphilic polymer idea here in this paper, together with nanoscale materials design and synthesis, it is possible to improve the cycle life up to 10,000 cycles," Cui told Phys.org. "My group is working on this. Our recent results on nanomaterials design already improved to 1000 cycles."

In the future, Cui think Li-S batteries will give Li-ion batteries some serious competition.

"The Li-S batteries become pretty promising for electric vehicles," he said. "The life cycle needs to improve further. The lithium metal anodes' safety problem needs to be solved. It is possible to get around Li metal anodes with Si anodes."

Explore further: Scanning tunnelling microscopy: Computer simulations sharpen insights into molecules

More information: Guangyuan Zheng, et al. "Amphiphilic Surface Modification of Hollow Carbon Nanofibers for Improved Cycle Life of Lithium Sulfur Batteries." Nano Letters. DOI: 10.1021/nl304795g

Journal reference: Nano Letters search and more info website

4.7 /5 (34 votes)

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dschlink
5 / 5 (2) Feb 25, 2013
Good news, but I'll point out that Sion achieved 300 full depth discharge cycles in 2002.
praos
2.7 / 5 (7) Feb 25, 2013
300 cycles do not sound impressive; however in a EV with range of 300mi (made possible by such battery) this makes 90,000mi, corresponding to 6-7y of average US car use. And after that you have a car with range of "only" 240mi (or a second-hand battery of 60 kWh, inuf to drive your home for 3 days). But with 1000 cycles...
javjav
5 / 5 (5) Feb 25, 2013
And after that you have a car with range of "only" 240mi

After that you could simply replace the battery, as it could be much cheaper than Li-ion.
trekgeek1
not rated yet Feb 25, 2013
a Li-S battery that can retain more than 80% of its 1180 mAh/g capacity over 300 cycles, with the potential for similar capacity retention over thousands of cycles.


That's kind of a strange unit for capacity since it reduces to Charge/Mass. That's fine if they would have given the voltage these things are typically supplying, which relates charge to energy. I mean, can I make a cell with a 100 Volt potential? What's the limit on the technology?
gwrede
5 / 5 (1) Feb 25, 2013
trekgeek: Car makers are interested in what the battery will weigh. That's why they use charge/mass. And if you want more voltage, you connect cells in series, that is, end to end.
topkill
4 / 5 (4) Feb 25, 2013
No, what trekgeek is asking is the right question. You don't know how much energy it holds unless they also tell you the voltage as well as the Ah. Of course, you have to pair this with a compatible anode and then know the voltage of the cell (the potential difference between them). Most lihthium family batteries end up around 3.2V
The really exciting thing about this is that it's the cathode with such a high energy density. They usually lag behind the anode in energy density and hold back the overall cell density. But if you could pair this with a similar anode and together they averaged around 1,000mAh/g with ~3.0V, then even factoring in the electrolyte and packaging materials, you could get up to around 1,500Wh/kg for a battery pack.
But that's all fantasy with a lot of things that would have to be best case and 5 years minimum to hit the market IF everything went right (matching anode, electrolyte, safe, easy/cheap to manufacture, good cycle life for the cell, temp, etc)
uhjim
2.6 / 5 (5) Feb 25, 2013
It appears to be that human beings have bad battery karma. this leads me to suspect that the sun will begin to expire 20% sooner than expected

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