We live in a world hungry for electricity. New factories, new consumer electronics, new cars fueled by electrons. Engineers are racing to find better ways to build and power machines, pushing energy efficiency up incrementally as they go. Still, the U.S. Energy Information Agency forecasts that global demand for electricity will grow 2.3 percent per year through 2035.
Busses, trains, automobiles and aircraft are either starting to move off of fossil fuels for direct electrical motive energy or have more systems that draw current. And a growing stock of buildings are being energized using intermittent power sources like wind and solar. All of this demand is highlighting a major obstacle—batteries need to get better at safely storing large amounts of energy to power our modern world.
Without a major innovation in energy storage, much of the promise of greening aviation, militaries, power generation and other industries will be stuck in the mire.
That’s the challenge Oak Ridge National Laboratory materials scientist Chengdu Liang and his team have been working on since 2007. Now they say they have designed and successfully tested a new battery that can store four times the energy as conventional lithium-ion versions.
“Our battery is the next generation of energy storage,” Liang tells Txchnologist. “The materials we developed will be the foundation for batteries for the next 30 to 50 years. It will make a big impact.”
Engineering compounds to hold more energy
He says his team began working on improving storage capacity by first looking to conventional battery fabrication approaches, which use liquid electrolytes like those found in car batteries to efficiently conduct electrons between cathodes and anodes.
But they were unable to figure out how to increase energy storage using liquid, so they began researching all-solid batteries using lithium and sulfur-rich compounds. It was an approach that had been investigated before but never advanced because of technical problems. So the group designed new molecules that grew crystal structures allowing better conductivity and energy storage throughout the battery. “Our approach is a complete change from the current battery concept of two electrodes joined by a liquid electrolyte, which has been used over the last 150 to 200 years,” he says.
They manipulated lithium and elemental sulfur, a common byproduct of industrial processes, into compounds called lithium polysulfidophosphates. The work was published this month in the respected chemistry journal Angewandte Chemie International Edition.
“Sulfur is practically free,” Liang says. “Not only does it store much more energy than the metallic compounds used in lithium-ion battery cathodes, but a lithium-sulfur device could help recycle a waste product into a useful technology.”
The researchers demonstrated that their device could maintain a capacity of 1,200 milliamp-hours (mAh) per gram through 300 charge-discharge cycles at 140 degrees Fahrenheit. Traditional lithium-ion batteries, in comparison, have capacities between 140-170 mAh/g.
They explained in a lab release that because lithium-sulfur batteries deliver about half the voltage of lithium-ion versions, this eight-fold increase in capacity demonstrated in their new battery translates into four times the gravimetric energy density of lithium-ion technologies. “Our battery design has real potential to reduce cost, increase energy density and improve safety compared with existing lithium-ion technologies,” he says.
Having more energy available when its needed isn’t the only reason to take note of this potential Oak Ridge development, though.
Major safety upgrade
Nobody aboard All Nippon Airways flight NH 692 had a very good day on Jan. 16, 2013. The aircraft, a brand new Boeing 787, took off from southwestern Japan at 8:10 am local time en route to Tokyo. But about 30 minutes into the routine flight, the pilots received a warning that something was amiss in one of the plane’s electrical compartments.
“There was a battery alert in the cockpit and there was an odd smell detected in the cockpit and cabin, and [the pilot] decided to make an emergency landing,” said Osamu Shinobe, an ANA vice president, during a press conference following the event.
The plane landed safely at a nearby airport, with only a handful of passengers sustaining minor injuries. During an investigation into a similar incident aboard another 787—an empty Japan Airlines aircraft in Boston—the U.S. National Transportation Safety Board said a malfunctioning lithium-ion battery filled with a flammable liquid electrolyte had short-circuited and caught fire.
Through a series of events within the new planes’ batteries, which were part of the aircraft’s auxiliary power unit and have since been redesigned, a temperature increase fueled a positive feedback loop that led to what is called a thermal runaway and a dangerous situation.
“We know the lithium-ion battery experienced a thermal runaway, we know there were short circuits, and we know there was a fire,” NTSB Chairwoman Deborah Hersman said during a news conference about the Boston incident.
While the NTSB is still investigating the Boston Japan Airlines incident, the flammable liquid ingredients inside conventional lithium-ion batteries turn a major problem into one that is potentially deadly.
The issue is another reason that the Oak Ridge team focused on using a much safer mixture of solid sulfur and lithium.
“That liquid electrolyte solution is highly flammable,” Liang says. “That’s why we are looking to develop materials that are resistant to fire. We realized that if we used a good solid, then we could get the same or better performance in terms of safety along with performance.”
Their battery still needs major work—it has only been experimentally demonstrated in the lab. More engineering, design and manufacturing expertise need to get it in shape before commercialization. But Liang can already see its application in large-scale storage for vehicles, military applications, solar and wind farms, and in the telecommunications industry.
“We solved the science problem,” Liang says. “We used fundamental research to understand a scientific phenomenon, identified the problem and then created the right material to solve that problem. Now we invite engineers to play on this new playground.”