Lithium-ion cells that use polymer electrolytes can be affordably packaged in compact, flexible pouches
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 today.
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-Québec 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 practical.”
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.”