Lithium-ion technology is still the gold standard for energy storage as demonstrated by the popularity of the new Powerwall battery, Tesla Energy’s much-publicized foray into Li-ion energy storage for homes and businesses. However, some new technologies are sneaking up behind. In the latest development, lithium-sulfur batteries could benefit from a new “designer carbon” engineered by a team of researchers at Stanford University.
Why Natural Is Not Better, Energy Storage Edition
The new designer carbon material could have a variety of applications, but the Stanford University team has zeroed in on the energy storage potential, particularly in respect to lithium-sulfur (Li-S) batteries.
The new material is actually a synthetic form of bio-based activated carbon. For those of you new to the topic, activated carbon is a common material that shows up in water filters and deodorizers, among many other things — but not energy storage devices, at least not yet.
Inexpensive forms of activated carbon are typically made from coconut shells, which involves a lot of high-temperature processing and chemical finishing. The result is a material rich in nanoscale pores, which gives it a high surface area ideal for storing electrical charges.
However, this “natural” form of activated carbon falls flat in terms of transporting a charge, partly because there is little connectivity between the pores. Here’s lead researcher Zhenan Bao describing the problem:
With activated carbon, there’s no way to control pore connectivity. Also, lots of impurities from the coconut shells and other raw starting materials get carried into the carbon. As a refrigerator deodorant, conventional activated carbon is fine, but it doesn’t provide high enough performance for electronic devices and energy-storage applications.
The Designer Carbon Solution
As a workaround, the Stanford team created its own synthetic sheets of carbon from a hydrogel polymer (hydrogel is fancyspeak for a class of super-absorbing “smart” materials). To activate the material, they added potassium hydroxide, which also increased its surface area.
The result is a carbon material with characteristics that can be controlled in two ways: by using different polymers and organic linkers, and by changing the temperature of the fabrication process.
Here are a couple of snippets from the new study:
For example, raising the processing temperature from 750 degrees Fahrenheit (400 degrees Celsius) to 1,650 F (900 C) resulted in a 10-fold increase in pore volume.
Subsequent processing produced carbon material with a record-high surface area of 4,073 square meters per gram – the equivalent of three American football fields packed into an ounce of carbon. The maximum surface area achieved with conventional activated carbon is about 3,000 square meters per gram.
The End Of The Lithium-Ion Era
Li-S energy storage has important advantages over Li-ion in terms of cost, energy density, and toxicity, but until recently, some major drawbacks have stymied the development of Li-S batteries.
One solution crossed our radar back in 2013, when researchers at Oak Ridge National Laboratory developed a sulfur-enriched cathode (our sister site Gas2.org also took note).
In other developments, the University of Arizona has also been developing a method for converting waste sulfur to a lightweight plastic that could be used in EV batteries. Last December, researchers at Cambridge University came up with a graphene-based solution, and earlier this year, Drexel University announced that it has been leveraging its experience with MAX phase ceramics to push the Li-S envelope.
The new Stanford findings add more fuel to the energy storage findings. The team tested its new designer carbon material under real-world conditions in lithium-sulfur batteries and supercapacitors (supercapacitors are energy storage devices that charge and discharge rapidly).
For supercapacitors, the results were “dramatic,” with a threefold increase in conductivity compared to electrodes made with conventional activated carbon. Power delivery and stability also improved.
More to the point, the results showed a promising pathway to improving Li-S battery performance, as the designer carbon was able to trap lithium polysulfides, an undesirable byproduct from the interaction of lithium and sulfur.
The new material’s relatively low cost and easy fabrication method are added pluses. You can get all the details from the published study in ACS Central Science under the title “Ultrahigh Surface Area Three-Dimensional Porous Graphitic Carbon from Conjugated Polymeric Molecular Framework.”
You might not see much in the way of competition for Li-ion market share yet, but stay tuned.
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Photo Credit (cropped): New “designer carbon,” by John To and Zheng Chen, Stanford University.
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