It takes years to verify that a new battery technology will last for the life of a hybrid or electric car. That means battery materials that might cost less and store more energy than today's batteries are languishing on lab benches.
A new way to test lithium-ion batteries could cut that time to a few weeks instead of a few years, eliminating a key bottleneck that's keeping battery costs high and storage capacities low.
By accurately measuring how efficiently experimental batteries store and deliver an electrical charge, Jeff Dahn of Dalhousie University can predict how many times battery cells can be charged and discharged--known as the cycle life of the battery. Dahn, a professor of physics and chemistry, is also trying to demonstrate that the method can predict how long a battery will last on the shelf--known as calendar life.
Battery tester: This system charges and discharges 60 battery cells and predicts how long they will last.
Credit: Jeff Dahn, Dalhousie
Credit: Jeff Dahn, Dalhousie
Together, cycle life and calendar life determine how long a battery will be useful. They're essential for determining, for example, how big the battery pack needs to be to store the advertised amount of energy throughout the life of the car.
The technique has caught the attention of automakers, which are trying to validate and use it, particularly as a tool for predicting cycle life. It could also allow academics, who have fewer resources than automakers, to develop battery materials with real commercial potential. "We think this technique could be very useful," says Masaki Matsui, manager of materials research development at Toyota Research Institute of North America. He says it will identify problems with materials very early in battery development, allowing researchers to quickly sort through combinations of battery electrodes and electrolytes.
A panoply of things can go wrong in a battery. The key insight of Dahn's approach is that many such snags can show up in a single test--the measurement of the difference between the amount of charge that goes into a battery during charging and the amount that comes out when it's discharged (also called coulombic efficiency). If less charge comes out than goes in, that energy is being wasted by unwanted reactions within the battery. These losses add up: with successive cycles, the battery returns less and less charge until eventually it isn't usable.
Dahn has built a battery-charging system that can detect very small losses of charge, identifying in a few weeks the presence of life-shortening reactions that otherwise wouldn't show up until after months or years of testing. Dahn has used the technique to identify subtle changes in chemistry that can increase the cycle life of one type of battery up to sixfold.
Such accurate testing wasn't necessary for lithium-ion batteries when they were used almost entirely for portable electronics that have to last only a few years. But now batteries are being designed to power electric vehicles or to store energy from solar panels, and they must last 10, 15, 20 years or longer. Over such spans, even a tiny inefficiency can lead to big problems.
What's more, battery manufacturers are mixing up ever-more-complex electrolyte cocktails, each part of which could significantly change a battery's life. "The electrolytes have something like 10 components. Then you want to evaluate more additives. It's just a nightmare," Dahn says. With conventional measuring techniques, he says, "It's hard to know whether a change you've made is good or bad without doing a test that's longer than your career." By accurately measuring coulombic efficiency, he says, "in a few weeks, you've got everything."
Mark Mathias, manager of General Motors's electrochemical energy research lab in New York, isn't completely convinced. "What Jeff's doing is a very good diagnostic. I agree we should be using it, but it's a tool, not a panacea," he says. For one thing, it doesn't tell researchers what's going wrong in a cell. Mathias says researchers need a much better understanding of the underlying causes for efficiency losses. That would help them solve the problems that Dahn's testing identifies.
Mathias also isn't sure the test will prove reliable for measuring calendar life. It could be used to help evaluate batteries, he says, but "the unfortunate reality is, we can't know for sure that we have an accelerated test that mimics 10 years unless we've tested for 10 years," he says.
Dahn says that right now his equipment is accurate enough to tell researchers whether a particular change to battery chemistry will make the difference between it lasting 500 charging cycles (needed for a few years of driving an electric vehicle) or 1,000. He's now working with an equipment manufacturer to improve the process for accurate predictions to about 10,000 cycles.
By Kevin Bullis
From Technology Review
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