Improved Power for Rested Batteries

Improved Power for Rested Batteries

A new study suggests that letting discharged lithium metal batteries take some time to rest can significantly improve their overall cycle life.
The promise of electric vehicles (EVs) depends on the automotive industry’s ability to extend life of vehicle batteries. Today, conventional lithium-ion batteries can only get you 200 to 300 miles on a single charge, making them less than ideal for the average American driver. Advances in lithium metal battery technologies can currently double that mileage, but their capacity to store energy quickly degrades after relatively few cycles of charging and discharging. That’s a problem, and one that needs to be solved for EVs to go mainstream.

Previous studies have shown that this degradation is due to a lithium corrosion effect, said Wenbo Zhang, a battery scientist in Yi Cui’s laboratory in the department of materials science and engineering at Stanford University’s School of Engineering in Palo Alto, Calif. When the battery is not fully discharged, but is at rest, micron-sized bits of lithium metal start causing troublesome reactions.

Tiny bits of dead lithium on the copper mesh from a lithium metal battery, magnified. Image: Wenbo Zhang/Stanford University
“Lithium is such a reactive metal, and it will react with the rest of the electrolytes in the battery, forming a layer we call the solid electrolyte interphase (SEI),” he said. “You can think of it as like a type of rust on the lithium. And this layer gets thicker over time and it kind of eats away at your battery capacity so that you lose range at a very fast rate.”

Material scientists have been looking for ways to address this issue, and Zhang said many have relied on conventional tests where they continuously cycle battery cells, charging and then immediately discharging the power source over and over again during their research studies.

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“We do that because it saves us time. We can say, okay, I’ve done X number of charges and that can be extrapolated out to estimate the performance of actual batteries,” he explained. “The problem is: that’s not how we really use these batteries in EVs. We drive for an hour, maybe two. Then you charge the battery overnight if you are doing the slow charge. Maybe 20 minutes if you are doing the fast charge option. But for the rest of the time, the car, and the battery, is just sitting there at rest. So, we started thinking about exploring what happens in a more realistic way.”

That inspiration to look at real-world conditions helped Zhang, Cui, and colleagues unearth a simple and low-cost solution to improve battery performance and cycle life: fully discharging the battery and then just leaving it alone for several hours.

“It all started with us thinking we could move the battery to a completely discharged state to get rid of the corrosion effect,” Zhang said. “We discharged the battery to study what happens to the SEI layer. To our surprise, which was co-discovered by my co-author Philaphon Sayavong, who was exploring SEI evolution, we saw that the corrosion was disappearing. And then, from my end, we saw we were getting better performance in the battery after it rested in the discharged state.”

By just letting the discharged battery rest for a single hour, the SEI layer dissolved to the point where, when the battery was later recharged, the previously disconnected pieces of lithium were able to reconnect with the anode, bringing it back to life.

“This was lithium that we thought was unrecoverable—it was isolated and couldn’t generate power anymore,” Zhang said. “But with just some rest, we brought it back. Not only could we bring back lost capacity, but we could also improve the battery’s performance.”
Image: Wenbo Zhang/Stanford University
With the knowledge that lithium metal batteries can be restored with rest, Zhang and colleagues are now focused on discovering other ways they could help recover lost capacity in these important power sources.

“The main avenues of capacity decay in lithium metal batteries are the formation of the SEI layer and the formation of these bits of isolated lithium. Whatever you lose in capacity usually ends up in one of these two byproducts,” he said. “Our thinking is that we can find alternative ways to recover this capacity. We are currently doing research into using heating as a way to recover capacity, because if you heat lithium, you can make it softer and also increase the pressure conditions within the cell. So, you can kind of compress it together while it’s softened and increase the chance of reconnection as well.”

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But, he added, there is still a lot of work to be done. Batteries are “extremely complicated,” and hundreds of factors influence their performance and capacity. Any improvement in one factor often leads to a trade-off in another area. Still, he said, this discovery is an easy, low-cost solution to recover capacity without the need for new materials or to manufacture new equipment. That’s a big benefit to the EV industry.

“All you have to do is change the battery management system to completely discharge your battery one at a time, instead of evenly throughout the module,” he said. “EVs don’t run on just one giant battery. You have a dozen or so different battery modules. So, until we come up with a different battery option, it’s possible to completely discharge a single one of them to let it rest and still have another 10 or 11 that allows the vehicle to run without affecting your vehicle’s overall performance. That’s one little thing that could make a big difference.”

Kayt Sukel is a technology writer in Houston.

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