Imagine a
battery that truly does keep on going and going—and not for just a few years,
but close to decades.
At the
U.S. Department of Energy’s (DOE) Argonne National Laboratory, materials
scientist Daniel Abraham works to do just that for lithium-ion batteries. These
are the kind that power cell phones, laptops, and the next generation of
plug-in hybrid or all-electric cars. Abraham and his colleagues are working to
extend battery life, while simultaneously trying to increase storage capacity.
“In
cell phones, you can get away with a battery that lasts only a couple of
years,” he says. “For vehicles, you need a battery that lasts 10 to
15 years, which is the typical lifetime of a car.”
To extend
batteries’ working lives, researchers need to understand how and why batteries
fail.
“I
think of it as solving a series of mysteries,” Abraham says. “How did
it happen? How can we prevent it from happening again? To find out, we often
take apart the battery and examine its contents for clues using an array of
diagnostic tools.
As it
turns out, there are many ways for batteries to fail.
A
lithium-ion battery contains four basic components: a positive cathode, a
negative anode, an electrolyte that allows lithium ions to flow between them;
and a separator that keeps the electrodes apart to prevent short circuits.
When you
charge the battery, the current from the electrical outlet forces lithium ions
to move from the cathode to the anode. This converts electrical energy from the
outlet into stored chemical energy. When you unplug the battery and begin to
use it, the lithium ions flow back to the cathode; the stored chemical energy
is converted into a stream of electrons to power the device.
Of course,
in a complex system, many things can go wrong.
Both the
cathode and anode are composed of tiny particles glued together with a chemical
“binder” and painted on over a metallic sheet. Sometimes the binder
fails and the composite comes apart; sometimes the entire layer peels off the
sheet.
The anode
is typically made up of long planes of graphite, called graphene. Lithium ions
arrange themselves between the graphene planes, making them expand and
contract—and eventually some of the planes can crack. The liquid electrolyte
also eats away at the edges of the graphite. Buildup of a protective layer
between the electrolyte and the graphite can prevent corrosion, but the layer
is built using precious lithium ions, taking them out of circulation and
lowering battery performance.
Meanwhile,
the cathode is typically made up of lithium-bearing oxide or phosphate
particles. The buildup of electrolyte reaction products can create obstacles to
ion motion, draining power performance. Or changes in the particles’ crystal
structure can siphon away energy during battery operation.
More
seriously, in rare cases lithium batteries can catch fire or even explode.
Quality
control matters more in lithium-ion batteries than in other battery systems. Battery packs contain individual cells that are
electrically linked together. But if one cell is faulty and doesn’t perform as
well as the others, it’s more than inefficient: it’s dangerous.
“Say
the faulty cell has only half the capacity of the others,” Abraham says.
“When I plug in to charge, that one cell will fill up faster than the
others, and it begins to overcharge. Remember that charging is just supplying
electrical energy into a battery. If I keep adding energy, the electricity has
nowhere to go, and it’s transformed into heat. The heat eventually exceeds the
flammability threshold of the electrolyte, and the cell bursts into
flame.”
Thus, the
well publicized recalls of lithium-ion batteries in recent years. Safety is
paramount, especially in batteries for cars, and considerable research is
directed at improving the intrinsic safety of battery materials.
“The
arrangement of atoms within the crystal structure plays a role in how the battery
performs,” Abraham explains. “What we are doing now is looking at
batteries in situ: that is,
actually watching the battery as it charges and drains, using powerful X-rays
from the Advanced Photon Source and electrons from the Electron Microscopy
Center. That’s the cutting edge of battery diagnostics.”
A
combination of methods allows researchers to examine every angle. For example,
microscopy methods help the team decipher the architecture and composition of
battery components, while X-ray techniques allow them to decode the arrangement
of individual atoms.
Armed with
an understanding of how things work or fail, the scientists set out to find
solutions to each problem. Modifying the elemental composition and size of
cathode particles can improve energy and power; a pre-made protective layer on
graphite can reduce corrosion; altering electrolyte chemistry can make the cell
more heat-tolerant; better manufacturing practices can reduce the incidence of
electrical shorts.
Now the
scientists have a new and hopefully improved battery. Now what?
“Obviously, the next step is to test it out,” Abraham says. “But
when you want to see how the battery performs over 15 years, you have a problem
on your hands: you can’t wait 15 years for the results.”
“Accelerated aging” is a way to simulate the results of 15 years
of wear and tear in just a few months or a year. Battery
testers charge and drain the cells over and over again at elevated
temperatures; the test conditions are carefully chosen to mimic the effect of
years of use. Using this method, scientists can cycle through testing,
diagnostics, and chemistry modifications, changing and re-testing generations
of batteries in just a few years.
