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.
http://cleantechnica.com/2015/05/31/new-designer-energy-storage-breakthrough-packs-3-football-fields-1-ounce-carbon/