Lithium Ion Battery
Lithium Ion Battery

Koffi Pierre Yao of the University of Delaware is investigating the inner workings of rechargeable lithium ion batteries with a view to finding a way to cut charging times for "mobile" devices including smart phones and electric cars.

Researchers have been trying to optimize lithium-ion batteries for years so that refueling an electric vehicle might be done within minutes rather than overnight.

"Usually people will make an electrode, test it, make another one, test it, and so on, and it's kind of a serial process," explains Yao. He has taken a different tack. He is using physical probes to look inside batteries while they are operating to try and glean a direct physical understanding of lithium ion flow within. 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.

Now, writing in the journal Energy & Environmental Science, he and his colleagues show how they used X-ray studies to obtain a micrometer-scale movie of how lithium is redistributed within the electrode while the battery is running. "We put an industrial-grade battery under an X-ray beam and mapped the distribution of the lithium within the electrodes," Yao explains. [Yao, K.P.C. et al. Energy Environ. Sci. (2019) 12, 656-665; DOI: 10.1039/C8EE02373E]

The team already knew that lithium does not move homogeneously within the electrode but what surprised them was the degree of inhomogeneous scattering of these ions. They hope the new insights can be exploited to reduce testing times and so accelerate the research and development process. In a separate paper (Yao, K.P.C. et al. Adv. Energy Mater. 2019); DOI: 10.1002/aenm.201803380] they again used X-rays to quantify the activity in a silicon-graphite electrode. Conventional lithium-ion batteries usually contain graphite, but silicon offers benefits over graphite.

"We're interested in silicon because it can increase the capacity of the electrode by a factor of ten compared to graphite," Yao says. 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 adds.

The team has now provided what they say is a clearer picture of when the silicon and graphite play host to lithium at a given point in time. They suggest that they can now experiment to find ways to manipulate this pattern and so stabilize the charging cycle."