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Lithium-Sulfur Batteries Longevity Hack February 25, 2013

(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 (copied from Phys.org)

Lithium-ion cells are currently the most commercially successful battery type, but their low energy density makes for poor long-distance travel, and they can cost about half the price of electric cars they power. Lithium-sulfur (Li-S) batteries, on the other hand, have a very high energy density that allows them to store more energy than Li-ion batteries and therefore provide a nice long trip (and are much cheaper than Li-ion batteries). Why are we using Li-ion batteries?

Lithium-Sulfur batteries tend to lose charging capacity pretty quickly, dropping to a fraction of their original energy storage capacity in a very few charge-discharge cycles (like in the tens.  Not so good). Yi Cui, a prolific (or at least his graduate students are really busy) professor of materials science and engineering 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. This is really a huge leap in battery lifetime, like 10X. 

The Transmission Electron Microscope scans above show what they did.  The leftmost  shows a typical cathode (sulfur); the next shows a discharged sulfur cathode.  The little hollow space there is where the sulfur has drawn away from the nanofiber wall that supports it, making contact (and recharging) difficult.  Several discharges will result in more mechanical damage like this.  What the researchers did was to add a polymer to bind the sulfur more completely to the the inner surface of the nanofiber which holds it, making it available for charging and preventing mechanical stress.

Sulfur cathodes containing these 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. However, Li-ion batteries may have lifespans approaching 10,000 cycles, which electric vehicles require to avoid swapping batteries in just a few years (remember, batteries are HALF the cost of an electric car). Cui thinks that Li-S batteries can close this gap in the near 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 says. "My group is working on this. Our recent results on nanomaterials design already improved to 1000 cycles."

Homework: 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