“Iron fluoride has the potential to triple the amount of energy a conventional lithium-ion battery can store,” explained Song Jin, a UW–Madison professor of chemistry and Wisconsin Energy Institute affiliate. “However, we have yet to tap its true potential.”

Graduate student Linsen Li worked with Jin and other collaborators to perform experiments with a state-of-the-art transmission X-ray microscope at the National Synchrotron Light Source at Brookhaven. The researchers collected chemical maps from actual coin cell batteries filled with iron fluoride during battery cycling to determine how well they perform. The results are published today in the journal Nature Communications.

“In the past, we weren’t able to truly understand what is happening to iron fluoride during battery reactions because other battery components were getting in the way of getting a precise image,” said Li.

By accounting for the background signals that would otherwise confuse the image, Li was able to accurately visualize and measure, at the nanoscale, the chemical changes iron fluoride undergoes to store and discharge energy.

Using iron fluoride in rechargeable lithium ion batteries has presented scientists with two challenges. The first is that it does not recharge well in its current form.

“This would be like your smart phone only charging half as much the first time, and even less thereafter,” said Li. “Consumers would rather have a battery that charges consistently through hundreds of charges.”

By examining iron fluoride transformation in batteries at the nanoscale, Jin and Li’s X-ray imaging method pinpoints each individual reaction to understand why capacity decay may be occurring.



Chemical phase map showing how the electrochemical discharge of iron fluoride microwires proceeded from 0 percent discharge (left), to 50 percent (middle), to 95 percent (right).