A Car Battery at Half the Price

Battery prototype: Two sludge-like electrode materials are fed into the device shown here. The anode material flows into the top half, and the cathode flows into the bottom. Lithium ions pass from one material to the other, and electrons flow through the black and red leads.
Credit: Yet-Ming Chiang

ENERGY

A Car Battery at Half the Price

A startup hopes to commercialize a novel design that features fluid electrodes.

Last year, the battery startup A123 Systems spun out another company, called 24M, to develop a new kind of battery meant to make electric vehicles go farther and cost less. Now a research paper published inAdvanced Energy Materials reveals the first details about how that battery would work. It also addresses the challenges in bringing the battery to market.

A big problem with the lithium-ion batteries used in electric vehicles and plug-in hybrids is that only about 25 percent of the battery's volume is taken up by materials that store energy. The rest is made up of inactive materials, such as packaging, conductive foils, and glues, which make the batteries bulky and account for a significant part of the cost. 

24M intends to greatly reduce the inactive material in a battery.  According to estimates in the new paper, its batteries could achieve almost twice the energy densities of today's vehicle battery packs. Batteries with a higher energy density would be smaller and cheaper, which means electric and hybrid cars would be less expensive. The paper estimates that the batteries could cost as little as $250 per kilowatt hour—less than half what they cost now.

A conventional battery pack is made up of hundreds of cells. Each cell contains a stack of many thin, solid electrodes. These electrodes are paired with metal foil current collectors and separated from each other by plastic films. Increasing the energy storage requires adding more layers of electrode material—which in turn requires more layers of metal foil and plastic film.

24M's design makes it possible to increase energy storage without the extra metal foil and plastic film. The key difference is that the electrodes are not solid films stacked in a cell, but sludge-like materials stored in tanks—one for the positive electrode material and another for the negative electrode.

The materials are pumped from the tanks into a small device, where they move through channels carved into blocks of metal. As this happens, ions move from one electrode to the other through the same kind of separator material used in a conventional battery. Electrons make their way out of the material to an external circuit. In this design, increasing energy storage is as simple as increasing the size of the storage tanks—the device that allows the electrodes to interact stays the same size. The design also does away with the need to wire together hundreds of cells to achieve adequate energy storage.

The new battery is similar to something called a flow battery, in which two electrolytes are pumped past each other. But conventional flow batteries are about 10 times larger than the new design because they use dilute energy storage solutions, which makes them impractical for use in cars.

The researchers, led by Yet-Ming Chiang, a professor of materials science at MIT, and a founder of both A123 Systems and 24M, tested various materials for the electrodes, including lithium cobalt oxide, which is commonly used in laptop batteries. They demonstrated that the device can charge and discharge at the rates needed in electric vehicles, Chiang says.

The paper also describes how the researchers address one of the biggest challenges of the design: pulling the electric charge out of the sludge. In an ordinary lithium ion cell, the electrons make their way by jumping through the connected conductive particles in the solid electrode until they reach a current collector. In the new battery, the electrons won't flow through the electrolyte. So Chiang and colleagues mixed nanoscale carbon particles into the sludge; the particles spontaneously form interconnected networks in the fluid to provide pathways for the electrons to escape.

Challenges remain before the battery can be commercialized. The electrical conductivity is still about 100 times less than it should be in a practical system, Chiang says. He's also working on increasing the concentration of active materials in the sludge.

Jeff Dahn, a professor of physics and chemistry at Dalhousie University, notes that to achieve the power levels needed to propel a car, the electrochemical cell would still need to be large: the separator material would have to cover an area of about three meters by four meters. It could be cut into manageable pieces and stacked up, but this could make the system complicated, and even with this approach the cell could be bulky, he says.

"We're making good progress on the technology," says 24M CEO Throop Wilder. "The acceptance of the paper is strong validation of the fundamental principles that drive our development." 24M consists of about 20 employees, and has raised roughly $16 million.

"It's a very clever device," says Dahn. "I don't know if it will ever be more than an idea in a paper, but Chiang has surprised people before."