Charging Ahead: Taking the plug-in hybrid electric car farther on a single battery charge

The development of lithium-ion batteries for hybrid electric vehicles generated a paradigm shift by creating the potential for the full electrification of America's transportation network. Although the current generation of plug-in hybrid electric vehicles (PHEVs) cannot travel very far on a single charge, Argonne's transportation engineers have made groundbreaking discoveries that promise to bring the country ever closer to a zero-emissions transportation future.

Every six months, we’re reminded to change the batteries in our household appliances: smoke alarms, flashlights, and radios.  But what if you had to change the battery in your all-electric car just as often?  That's the problem that faces the generation of lithium-ion batteries used in today's all-electric vehicles.

Fortunately, researchers at Argonne, as part of the laboratory's multi-pronged effort to improve energy-storage technology, may have found a way to exponentially increase the calendar and cycle lifetimes of lithium-ion batteries.  Electric double-layer capacitors – conventionally known as "ultracapacitors" – have an energy density thousands of times greater than conventional capacitors and a power density hundreds of times greater than lithium-ion batteries. 

In an electric vehicle drivetrain, energy density provides sustained speed, while power density facilitates acceleration. "Energy density is what allows you to run a marathon; power density is what enables you to sprint," said Ted Bohn, an automotive engineer in Argonne's Center for Transportation Research. 

"Ultracapacitors aren't of much use just by themselves," he added, "but when you couple them with lithium batteries, they dramatically boost the performance of the entire vehicle."

When an electric vehicle merely needs to maintain a particular speed, it requires little of the battery's power density.  However, when the car needs to accelerate from a standstill to a cruising velocity, today's lithium-ion batteries must strain to provide the necessary oomph.  "Ultracapacitors," Bohn said, "give an electric vehicle that initial boost it needs to get going."

According to Bohn, commercially available lithium-ion batteries retail for thousands of dollars.  A purely electric car that lacked the extra power density supplied by ultracapacitors would permanently burn out its battery within a matter of months.  Without ultracapacitors, the cost of replacing such an expensive part during regular maintenance had complicated the development of economically competitive zero-emissions vehicles.

Today's hybrid cars recharge their batteries by transforming kinetic energy from the wheels into potential electrical energy as the driver brakes.  Conventional lithium-ion batteries, however, absorb this energy slowly and inefficiently.  By contrast, ultracapacitors, because of their immense internal surface area, soak up reclaimed energy like a sponge.  "By integrating the entire system," Bohn said, "we can drive down the cost.  When we can put these various electronic elements together, we'll transform an $8,000 battery into a $4,000 all-electric drivetrain system."

A New “Energy Frontier”

Ultracapacitors represent only one tool that Argonne's scientists and engineers are using to solve today's pressing challenges in electrical storage.  This past April, the U.S. Department of Energy announced that Argonne will host a new Energy Frontier Research Center (EFRC) devoted to energy storage research.  This new facility, which will be called the Center for Electrical Energy Storage (CEES), will bring together a team of 17 world-class researchers from Argonne, Northwestern University and the University of Illinois at Urbana-Champaign. 

At its core, CEES will draw on Argonne's vast expertise in lithium-ion battery technology.  "There is a tremendous amount of innovation that can happen just within lithium-ion technologies," said Mark Peters, deputy associate laboratory director for energy sciences and engineering.  Argonne's three-year-old Center for Nanoscale Materials offers a new set of capabilities and resources that scientists and engineers can use to dramatically improve lithium-ion technology.

Read more about Argonne's involvement in the American Reinvestment and Recovery Act here.

However, no matter how far energy storage technology may develop in the laboratory, basic scientists must connect with industrial partners in order for CEES discoveries to make their way into cars on the dealership lot.  The American Reinvestment and Recovery Act, passed this spring, allocated more than $2 billion for domestic battery manufacturing, and Argonne has already started building relationships with other institutions to work toward that goal. 

In April, the laboratory reached an agreement with the Commonwealth of Kentucky, the University of Kentucky and the University of Louisville to create a joint battery manufacturing R&D center that will be located in central Kentucky and at Argonne.  "Perhaps the biggest challenge in battery technology today is setting up a significant manufacturing presence in the United States," Peters said.  "The batteries that are currently in hybrid vehicles and planned for plug-in hybrid vehicles, like the Chevy Volt, and ultimately electric cars, all come from Asia; we can create a lot of jobs and save a lot of money by doing the same work domestically."

As it gradually weans America's transportation infrastructure from its dependence on fossil fuels, advanced storage technology also offers the opportunity for electrical consumers to transfer energy back to the electrical grid during periods of peak demand.  The wall outlet for charging a plug-in hybrid could easily form a two-way connection between the consumer and the grid; when the car's battery will not be needed for a certain length of time, the owner could sell the battery's energy back into the grid when power is priciest – say, during the mid-afternoon on a hot summer day – then refill the battery overnight when the price of electricity has fallen.  

"Research in new storage technologies and research in grid improvements have, until now, proceeded along two largely separate paths," Peters said.  "As we begin to electrify the transportation sector, we're faced with a great opportunity to bring together the benefits that both of these fields of study will produce."

— by Jared Sagoff

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For more information, please contact Jared Sagoff (630/252-5549 or jsagoff@anl.gov) at Argonne.