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.
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