The research offers a promising solution to the problem of sharp drop-offs in the output of wind and solar systems with minor changes in weather conditions. Image: Charles Cook/Creative Commons
doesn’t always shine and the breeze doesn’t always blow and therein lie perhaps
the biggest hurdles to making wind and solar power usable on a grand scale. If
only there were an efficient, durable, high-power, rechargeable battery we
could use to store large quantities of excess power generated on windy or sunny
days until we needed it. And as long as we’re fantasizing, let’s imagine the
battery is cheap to build, too.
Now Stanford University researchers have developed
part of that dream battery, a new electrode that employs crystalline
nanoparticles of a copper compound.
tests, the electrode survived 40,000 cycles of charging and discharging, after
which it could still be charged to more than 80% of its original charge
capacity. For comparison, the average lithium ion battery can handle about 400
charge/discharge cycles before it deteriorates too much to be of practical use.
rate of several cycles per day, this electrode would have a good 30 years of
useful life on the electrical grid,” says Colin Wessells, a graduate
student in materials science and engineering who is the lead author of a paper
describing the research, published in Nature
a breakthrough performance—a battery that will keep running for tens of
thousands of cycles and never fail,” says Yi Cui, an associate professor
of materials science and engineering, who is Wessell’s adviser and a coauthor
of the paper.
electrode’s durability derives from the atomic structure of the crystalline
copper hexacyanoferrate used to make it. The crystals have an open framework
that allows ions to easily go in and out without damaging the electrode. Most
batteries fail because of accumulated damage to an electrode’s crystal
ions can move so freely, the electrode’s cycle of charging and discharging is
extremely fast, which is important because the power you get out of a battery
is proportional to how fast you can discharge the electrode.
the benefit of the open structure, the researchers needed to use the right size
ions. Too big and the ions would tend to get stuck and could damage the crystal
structure when they moved in and out of the electrode. Too small and they might
end up sticking to one side of the open spaces between atoms, instead of easily
passing through. The right-sized ion turned out to be hydrated potassium, a
much better fit compared with other hydrated ions such as sodium and lithium.
perfectly—really, really nicely,” says Cui. “Potassium will just zoom
in and zoom out, so you can have an extremely high-power battery.”
The speed of
the electrode is further enhanced because the particles of electrode material
that Wessell synthesized are tiny even by nanoparticle standards—a mere 100
dimensions mean the ions don’t have to travel very far into the electrode to
react with active sites in a particle to charge the electrode to its maximum
capacity, or to get back out during discharge.
A lot of
recent research on batteries, including other work done by Cui’s research
group, has focused on lithium ion batteries, which have a high energy density—meaning
they hold a lot of charge for their size. That makes them great for portable
electronics such as laptop computers.
density really doesn’t matter as much when you’re talking about storage on the
power grid. You could have a battery as big as a house since it doesn’t need to
be portable. Cost is a greater concern.
Some of the
components in lithium ion batteries are expensive and no one knows for certain
that making the batteries on a scale for use in the power grid will ever be
decided we needed to develop a ‘new chemistry’ if we were going to make
low-cost batteries and battery electrodes for the power grid,” Wessells
researchers chose to use a water-based electrolyte, which Wessells describes as
“basically free compared to the cost of an organic electrolyte” such
as is used in lithium ion batteries. They made the battery electric materials
from readily available precursors such as iron, copper, carbon, and nitrogen—all
of which are extremely inexpensive compared with lithium.
significant limitation to the new electrode is that its chemical properties
cause it to be usable only as a high voltage electrode. But every battery needs
two electrodes—a high voltage cathode and a low voltage anode—in order to
create the voltage difference that produces electricity. The researchers need
to find another material to use for the anode before they can build an actual
But Cui says
they have already been investigating various materials for an anode and have
some promising candidates.
they haven’t constructed a full battery yet, the performance of the new
electrode is so superior to any other existing battery electrode that Robert
Huggins, an emeritus professor of materials science and engineering who worked
on the project, says the electrode “leads to a promising electrochemical
solution to the extremely important problem of the large number of sharp
drop-offs in the output of wind and solar systems” that result from events
as simple and commonplace as a cloud passing over a solar farm.
Wessells note that other electrode materials have been developed that show
tremendous promise in laboratory testing but would be difficult to produce commercially.
That should not be a problem with their electrode.
been able to readily synthesize the electrode material in gram quantities in
the laboratory. He says the process should easily be scaled up to commercial
levels of production.
chemicals in a flask and you get this electrode material. You can do that on
any scale,” he says.
“There are no
technical challenges to producing this on a big-enough scale to actually build
a real battery.”