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.”