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.