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Sustainable Energy

A Battery-Ultracapacitor Hybrid

A device for power tools may also help regenerative braking.

By combining the chemistries of ultracapacitors and lithium-ion batteries, a company called Ioxus has created a hybrid energy-storage device that could recharge power tools in minutes and might never need to be replaced. The company says future incarnations could perhaps be used to capture energy from braking vehicles.

Battercapacitor: A new energy storage device blends the chemistry of an ultracapacitor with that of a lithium-ion battery.

Ultracapacitors capture and release energy in seconds and can do so millions of times, but they store only about 5 percent as much energy as lithium-ion batteries. The hybrid can store more than twice the energy by volume of standard ultracapacitors. That’s still much less than a lithium-ion battery, but the hybrid can be recharged quickly over 20,000 times as against a few hundred cycles for a typical battery.

A power tool using the lithium-ion ultracapacitor would run for only a 15th as long as it would on a battery but would recharge in just a minute. “Our product is for weekend warriors who don’t use the power tool much every day” but want very fast charging, says Mark McGough, CEO of Ioxus. The company, which is based in Oneont, New York, already makes conventional ultracapacitors for hybrid-electric buses and for engine start-stop systems that are used to increase fuel economy in cars.

The hybrid energy-storage device consists of an etched aluminum film coated on one side with carbon slurry, which is similar to the electrode found in an ultracapacitor. The other electrode, on the other side of the film, is coated not with carbon but with a lithium-ion material, providing more energy-storage capacity. The film is wound into a cylinder to make the finished device.

Ultracapacitors are being tested in some city buses as a way to capture the energy generated by braking and quickly release it for reacceleration, an approach that promises to improve fuel efficiency. If the hybrid lithium-ion ultracapacitor can be scaled up, it could improve fuel efficiency further by storing more energy. But its cycle life will need to be improved, as vehicle breaking systems need to be recharged hundreds of thousands of times.

The concept of hybrid lithium-ion ultracapacitors has been around for 20 years, but there is more demand for other types of energy-storage devices, says Theodore Bohn, an engineer at Argonne National Laboratory’s Advanced Powertrain Research Facility.

Bohn says the hybrid technology could nonetheless be ideal for small, lightweight applications that would benefit from having some of the power advantage of ultracapacitors and some the energy advantage of a battery. “The hybrid is good for the power pulse and OK for the energy,” he says.

Only one other company—JSR Micro, in Tokyo—makes hybrid devices of this type, having brought them to market in 2009. The company says its device has three times the energy density of a conventional ultracapacitor and a cycle life of 100,000 recharges. Jeff Myron, a program manager at JSR Micro, says the device is mainly intended as a backup power supply in medical-imaging equipment.

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