Public Release: 

The secret life of batteries

Professor examines the properties of batteries so he can make them charge faster

University of Delaware

You probably use batteries every single day, but do you actually understand how they work? Koffi Pierre Yao, a new assistant professor of mechanical engineering at the University of Delaware, is uncovering novel insights about what happens inside the batteries that power our smartphones, laptops, and electric vehicles. He plans to use this knowledge to develop faster-charging batteries that make electric vehicles the go-to automobiles for drivers.

Several of today's electric vehicles, such as the Tesla Model 3 and Nissan Leaf, run on lithium-ion batteries. But it takes inconveniently too long to recharge those vehicles when you can fill up your gas tank in the time it takes to pick up gas-station coffee. In a lithium-ion battery, positively charged lithium ions move through the electrode to deliver energy.

Scientists all over the world do time-consuming research on lithium-ion batteries in an attempt to optimize these power units. "Usually people will make an electrode, test it, make another one, test it, and so on, and it's kind of a serial process," said Yao.

Instead, Yao uses physical probes to look inside batteries while they work and develop a direct physical understanding of how lithium ions flow within batteries. When a battery is charging, the lithium flows unevenly in a way that's difficult to measure. Yao started working on this while he was a postdoctoral associate at Argonne National Laboratory (ANL), a position he held from 2016 until 2018, when he joined UD's faculty.

In a new paper published in Energy & Environmental Science, a journal published by the Royal Society of Chemistry, Yao describes how he and his colleagues at ANL used X-rays to get a micron-scale movie of how lithium distributes within the electrode while lithium-ion batteries are running.

"We put an industrial-grade battery under an X-ray beam and mapped the distribution of the lithium within the electrodes," he said.

Yao and his colleagues knew that the lithium did not distribute homogeneously. Imagine a group of people running through a small doorway. It takes time for people to spread out into the interior of the room; therefore, there will be crowding at the entry point. That's similar to how lithium moves through the electrode. Still, Yao and his colleagues were surprised at the extent to which lithium scattered inhomogeneously.

The goal is to use this knowledge to reduce testing time and speed up the research and development (R&D) process for these batteries.

In another new paper published in Advanced Energy Materials, Yao describes how he and his colleagues used X-rays to quantify the activity in a silicon-graphite electrode. Cell phone batteries typically contain graphite, but silicon offers some potential benefits over graphite.

"We're interested in silicon because it can increase the capacity of the electrode by a factor of 10 compared to graphite," he said. However, silicon is less stable than graphite and degrades faster, so a blend of the two may prove to be a viable solution. "Some of the lithium goes into the graphite, and some goes into the silicon," he said.

Yao and his colleagues sought to discover exactly where the lithium ions traveled within this blended electrode.

"It's something people haven't previously been able to do in the literature," Yao said. "We provide a clear picture of which of silicon and graphite plays host to lithium at any point in time. Now we can go forward and manipulate this pattern to stabilize the cycling." This knowledge can help Yao in his quest to design novel particles to make faster-charging and higher energy batteries.

At UD, Yao plans to expand upon his research on batteries with his colleagues at the Center for Fuel Cells and Batteries and more. Yao received his master's and doctoral degrees in mechanical engineering from the Massachusetts Institute of Technology (MIT) and his bachelor's degree in mechanical engineering at UD. As an undergraduate at UD, he was mentored by Ajay Prasad, Engineering Alumni Distinguished Professor and Chair of Engineering, who introduced him to electric cars and electrochemistry, the science behind them.

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