On a dreary Tuesday morning last October, the battery compartment of an electric Tesla Model S car erupted into spectacular ﬂames on the side of a rain-soaked highway in Kent, Washington.
Just 10 months before, a battery on board an empty Boeing Dreamliner sitting on a Boston tarmac also caught ﬁre, ﬁlling the cockpit with smoke. During the same month, a battery ﬁre on board a second Dreamliner forced the plane to make an emergency landing in southern Japan.
The spate of high-proﬁle battery ﬁres over the lasts few years - which also includes ﬁres sparked by crash-testing Chevy Volts - has heightened concern about using lithium-ion batteries in cars and planes, or really, in any capacity where powerful collisions are possible or where a ﬁre could have catastrophic consequences. A pair of researchers at CU-Boulder is aiming to eliminate that concern.
Conrad Stoldt and Sehee Lee, both associate professors of mechanical engineering, have created an experimental battery that would vastly reduce the risk of the type of thermal runaway reaction that dogs today's lithium-ion batteries. Just as important, the same technological breakthrough would increase the battery's energy density, an improvement that has the potential to double the range of today's electric cars.
"The innovative battery we developed is safer, more energy dense and less expensive than batteries used in electric vehicles today, and we think it has the great potential to push the U.S. electric vehicle industry forward," Lee says.
The battery's innovation is in what it's missing: liquid. Standard commercial lithium-ion batteries - the workhorse rechargeables that power everything from smart phones to laptops to cars - have a liquid, and typically ﬂammable, electrolyte that allows the lithium ions to ﬂow through a barrier that separates the battery's two electrodes, creating a charge.
If the barrier is damaged the electrodes can short-circuit, kicking off a chain of chemical reactions. As each chemical reaction adds heat to the system, it enables chemical reactions that require warmer temperatures to occur, which in turn add more heat to the system, enabling further reactions that require even more heat.
The CU-Boulder battery is made entirely out of solid materials, including a ceramic electrolyte, which blocks the kind of chain reactions that cause thermal runaways from occurring. And because the battery is safer, Stoldt and Lee were able to use pure lithium metal for the battery's anode instead of the carbon-based material typically used.
Chemists have known that a lithium metal anode would improve a rechargeable battery's performance, but using it has always been too risky. Lithium is highly reactive and would further increase the risk of battery ﬁres. But in a solid-state battery, the use of lithium metal as an anode becomes practical.
Using lithium metal boosts the battery's energy density, but practically speaking, the battery's performance is also enhanced by the simple fact that it's safer. The batteries used in today's electric vehicles are wrapped in insulation, ensconced in protective casing, and cooled with vents and fans, all in an effort to prevent thermal runaway reactions.
The bulky safety systems for these batteries do work. The battery compartment in the Tesla shunted the ﬂames away from the passenger compartment, keeping the driver safe. A federal investigation found that Chevy Volts were no more prone to ﬁres than gasoline vehicles, and Boeing was able to beef up the safety systems surrounding its on-board batteries.
But all that extra packaging adds weight to the car and, therefore, taxes the batteries more quickly. Just by virtue of making the vehicle lighter, the solidstate battery developed at CU-Boulder could last longer, extending the vehicle's range. Together with the use of the lithium metal, the new battery has the potential to double the range of electric vehicles.
Multiple teams from across the globe also are working feverishly on solid state batteries. But there is a signiﬁcant innovation that sets the CU-Boulder battery apart. The advance is in the construction of the battery's cathode, which in the new design is a conglomerate of cathode particles held together with solid electrolyte material and infused with an additive that increases its electrical conductivity. The net effect is that the electrons can move more easily within the cathode.
The innovation helped Solid Power - the company spun off by Stoldt and Lee to work on the commercialization of the battery - win a highly competitive $3.4 million grant from the U.S. Department of Energy.