March 27, 2009
Better Lithium-ion Batteries
A startup says its solid polymer electrolytes will mean cheaper, more-
By Prachi Patel
A new incarnation of lithium-ion batteries based on solid polymers is
in the works. Berkeley, CA-based startup Seeo, Inc. says its lithium-
ion cells will be safer, longer-lasting, lighter, and cheaper than
current batteries. Seeo's batteries use thin films of polymer as the
electrolyte and high-energy-density, light-weight electrodes. Lawrence
Berkeley National Laboratory is now making and testing cells designed
by the University of California, Berkeley spinoff.
Lithium-ion batteries are used in cell phones and laptops because they
are smaller and lighter than other types of batteries. They are also
promising for electric and hybrid vehicles. However, conventional
materials and chemistries have stopped them from being used
extensively in cars.
Today's lithium-ion batteries use lithium cobalt oxide electrodes and
a liquid electrolyte, typically lithium salts dissolved in an organic
solvent. The electrode material can release oxygen when overcharged or
punctured, causing the flammable solvent to catch fire and the battery
to explode. Besides, "the charged electrodes are very reactive with
the liquid electrolyte, which reduces power and [cycle-life]," says
Khalil Amine, manager of the advanced battery technology group at
Argonne National Laboratory.
Seeo's key breakthrough is a solid polymer electrolyte. It is not
flammable and hence inherently safer. In addition, the battery will
retain more of its capacity over time because the polymer does not
react with the charged electrode. "Lifetime data suggests that
conventional lithium-ion systems lose about 40 percent capacity in 500
cycles," says Mohit Singh, the cofounder of Seeo. "We get a much
better cycle life. We can go through 1,000 cycles with less than 5
percent capacity loss."
For the negative electrode, or anode, the electrolyte also works with
lithium metal films, which are lighter than current anode materials.
That means the battery can provide more energy for the same weight.
Based on the battery's single cell, Seeo has calculated that it would
have an energy density of up to 300 watt-hours per kilogram, which is
50 percent greater than lithium-ion batteries that are on the market
Batteries with solid electrolytes have the added bonus of being
cheaper to manufacture, Amine says. While liquid electrolytes have to
be tightly sealed inside a laser-welded metal container, plastic
electrolytes can be packaged inside heat-sealed pouches.
The advantages of polymer materials have warranted research on polymer
electrolytes for more than three decades. In fact, lithium polymer
batteries are already found in radio-controlled cars and MP3 players.
But they use a polymer gel containing solvents, so, like liquid
electrolytes they carry the risks of fire or explosion and do not have
a very long life.
Making solid polymers that are as conductive as liquid electrolytes
has been difficult. In a charging battery, the electrolyte conducts
lithium ions from the positive electrode, or cathode, to the anode.
The higher the conductivity of the electrolyte, the faster the battery
charges. St. Paul, MN-based 3M and Montreal, Canada-based electricity
provider Hydro-Qubec have spent more than 10 years on solid-polymer
lithium batteries. "But you have to operate the polymer at 60 degrees
Celsius to improve conductivity," Amine says. "This is not very
The problem is that a polymer's conductivity and mechanical strength
do not go hand-in-hand. "If people tried to make polymers with high
ionic conductivity they would end up with a goop," Singh says.
Seeo has gotten around the problem by making films with block
copolymers: materials containing two linked polymer chains that self-
assemble into nanostructures. One of the polymers forms an array of
conductive cylinders that are embedded within the other polymer, which
serves as a hard matrix. Singh says the electrolyte film is robust and
is almost as conductive as liquid electrolytes.
Seeo's technology "has become very attractive" because of its claim of
a high-conductivity polymer, Amine says. However, "the lithium anode
could be a show-stopper." Lithium has a tendency to get roughened at
the surface and grow crystal dendrites that can reach the cathode and
short the battery. The company will need to do long-term tests to show
that its polymer is hard enough to block the dendrites.
Polymer electrolytes also have one big inherent disadvantage.
"Polymers will always be limited by lower ionic conductivity compared
to liquids," Singh says. This means that Seeo's battery would be
limited for use in laptops and electric vehicles. "But these polymers
wouldn't be able to address quick-charge applications like hybrid-
electric vehicles or power tools."