Hybrid batteries that charge faster
than conventional ones could have significantly better electrical
capacity and long-term stability when prepared with a gentle-sounding
way of making electrodes.
Called ion soft-landing, the high-precision
technique resulted in electrodes that could store a third more energy
and had twice the lifespan compared to those prepared by a conventional
method, the researchers report today in Nature Communications. Straightforward to set up, the method could eventually lead to cheaper, more powerful, longer-lasting rechargeable batteries.
"This is the first time anyone has been able to put together
a functioning battery using ion soft-landing," said chemist and
Laboratory Fellow Julia Laskin of the Department of Energy's Pacific
Northwest National Laboratory.
The advantages come from soft-landing's ability to build an
electrode surface very specifically with only the most desirable
molecules out of a complex mixture of raw components.
"It will help us unravel important scientific questions
about this energy storage technology, a hybrid between common lithium
rechargeable batteries and supercapacitors that have very high energy
density," said lead author, PNNL chemist Venkateshkumar Prabhakaran.
A different kind of hybrid
Although lithium ion rechargeable batteries are the go-to
technology for small electronic devices, they release their energy
slowly, which is why hybrid electric vehicles use gasoline for
accelerating, and take a long time to recharge, which makes electric
vehicles slower to "fill" than their gas-powered cousins.
One possible solution is a hybrid battery that crosses a
lithium battery's ability to hold a lot of charge for its size with a
fast-charging supercapacitor. PNNL chemists wanted to know if they could
make superior hybrid battery materials with a technology—called ion
soft-landing—that intricately controls the raw components during
preparation.
To find out, Laskin and colleagues created hybrid electrodes
by spraying a chemical known as POM, or polyoxometalate, onto
supercapacitor electrodes made of microscopically small carbon tubes.
Off-the-shelf POM has both positively and negatively charged parts
called ions, but only the negative ions are needed in hybrid electrodes.
Limited by its design, the conventional preparation
technique sprays both the positive and negative ions onto the carbon
nanotubes. Ion soft-landing, however, separates the charged parts and
only sets down the negative ions on the electrode surface. The question
that Laskin and team had was, do positive ions interfere with the
performance of hybrid electrodes?
To find out, the team made centimeter-sized square hybrid
batteries out of POM-carbon nanotube electrodes that sandwiched a
specially developed ionic liquid membrane between them.
"We had to design a membrane that separated the electrodes
and also served as the battery's electrolyte, which allows conduction of
ions," said Prabhakaran. "Most people know electrolytes as the liquid
sloshing around within a car battery. Ours was a solid gel."
They tested this mini-hybrid battery for how much energy it could
hold and how many cycles of charging and discharging it could handle
before petering out.
They compared soft-landing with conventionally made hybrid
batteries, which were made with a technique called electrospray
deposition. They used an off-the-shelf POM containing positively charged
sodium ions.
Cheers for the POMs
The team found that the POM hybrid electrodes made with
soft-landing had superior energy storage capacity. They could hold a
third more energy than the carbon nanotube supercapacitors by
themselves, which were included as a minimum performance benchmark. And
soft-landing hybrids held about 27 percent more energy than
conventionally made electrospray deposited electrodes.
To make sure the team was using the optimal amount of POM,
they made hybrid electrodes using different amounts and tested which one
resulted in the highest capacity. Soft-landing produced the highest
capacity overall using the lowest amount of POM. This indicated the
electrodes used the active material extremely efficiently. In
comparison, conventional, sodium-based POM electrodes required twice as
much POM material to reach their highest capacity.
The conventionally-made devices used more POM, but the team
couldn't count them out yet. They might in fact have a longer lifespan
than the soft-landing produced electrodes. To test that, the team
charged and discharged the hybrids 1,000 times and measured how long
they lasted.
As they did in the previous tests, the soft-landing-based devices
performed the best, losing only a few percent capacity after 1000
cycles. The naked supercapacitors came in second, and the sodium-based,
conventionally made devices lost about double the capacity of the
soft-landed devices. This suggests that the soft-landing method has the
potential to double the lifespan of these types of hybrid batteries.Looking good
The team was surprised that it took so little of the POM material to make such a big difference to the carbon nanotube supercapacitors. By weight, the amount of POM was just one-fifth of a percent of the amount of carbon nanotube material.
"The fact that the capacitance reaches a maximum with so
little POM, and then drops off with more, is remarkable," said Laskin.
"We didn't expect such a small amount of POM to be making such a large
contribution to the capacitance."
They decided to examine the structure of the electrodes
using powerful microscopes in EMSL, the Environmental Molecular Sciences
Laboratory, a DOE Office of Science User Facility at PNNL. They
compared soft-landing with the conventionally made, sodium-POM
electrodes.
Soft-landing created small discrete clusters of POM dotting
the carbon nanotubes, but the conventional method resulted in larger
clumps of POM clusters swamping out the nanotubes, aggregates up to ten
times the size of those made by soft-landing.
This result suggested to the researchers that removing the positive ions from the POM starting material allowed the negative ions to disperse evenly over the surface. As long as the positive ions
such as sodium remained, the POM and sodium appear to reform the
crystalline material and aggregate on the surface. This prevented much
of the POM from doing its job in the battery, thereby reducing capacity.
When the team zoomed out a little and viewed the nanotubes
from above, the conventionally made electrodes were covered in large
aggregates of POM. The soft-landed electrodes, however, were remarkably indistinguishable from the naked carbon nanotube supercapacitors.
In future research, the team wants to explore how to get the
carbon materials to accept more POM, which might increase capacity and
lifespan even further.
Explore further:
Carbon doped with nitrogen dramatically improves storage capacity of supercapacitors
More information:
Venkateshkumar Prabhakaran et al. Rational design of efficient
electrode–electrolyte interfaces for solid-state energy storage using
ion soft landing, Nature Communications (2016). DOI: 10.1038/NCOMMS11399
Post a Comment