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Shabbir Ahmed, assistant professor in South Dakota State University’s Department of Mechanical Engineering, is conducting unique research to better understand a technological invention that has defined the 21st century: lithium-ion batteries. 

In the late 1800s, miners extracted lithium from igneous rock deposits near Keystone, South Dakota. The Etta Mine was the first in the United States where lithium was unearthed, and doctors utilized the mineral to treat gout and mental illnesses, like mania and depression.

Today, lithium is almost exclusively used to make lithium-ion batteries — widely considered one of the most impactful technological inventions of the last 100 years. From iPhones to laptops to electric vehicles, the lithium-ion battery is the most powerful rechargeable battery commercially available and has effectively transformed how we communicate as a society. In the future, lithium-ion batteries are expected to be utilized even more for sustainable transportation and energy. 

But there are still considerable challenges related to lithium-ion batteries, and Shabbir Ahmed, an assistant professor in South Dakota State University's Department of Mechanical Engineering, is working toward a solution.

Is the gauge accurate?

A fuel gauge is found in most cars and is used to indicate the amount of a fuel in a tank. Fuel gauges provide a clear picture of how long a car has before it must be refilled. Electronic devices — like cell phones, laptops and tablets — have a similar mechanism.

Often found in the upper right-hand corner of the screen, the "state of charge" displays the power remaining in the device. Like the fuel gauge in the car, it represents how much energy is left in the battery system and is inferred by microprocessors in the devices using a combination of current and voltage.

The state of charge is displayed as a percentage of the battery's storage. For example, your cell phone shows it has 75% remaining. The question is: How accurate is that percentage?

According to Ahmed, the state of charge is not nearly as accurate as we may think.

"It's not exactly accurate for several reasons," Ahmed said. "It's just an approximation."

Another consideration is the battery's "state of health." Lithium-ion batteries undergo an aging process with use. For example, cell phone owners might notice that the charge on their phone doesn't last nearly as long after a few years of use. This aging concept can be measured as the battery's "state of health," or how much of the original battery's capacity remains. In general, electronic devices do not provide users with the state of health of their device's lithium-ion battery.

"My research is looking to more accurately predict the state of charge and the state of health through different types of measurements," Ahmed said.

For cell phones, laptops and chargers, knowing the precise state of charge or the state of health is not exactly essential information for using the device. It may be inconvenient, but it does not completely inhibit usage. But if lithium-ion batteries are going to take a more central role in transportation — say, for all-electric helicopters — knowing the exact state of charge and state of health is essential.

"For cell phones, this might not be that important," Ahmed said. "But for EV-powered transportation, like cars or helicopters, it is much more important."

Wave measurements

Lithium-ion batteries function as lithium ions shuffle between the positive and negative sides of the battery via the separator. When a device is "on" (discharging) and providing an electric current, the anode — negative side of the battery — releases lithium ions to the cathode — the positive side — generating a flow of electrons. When the device is plugged in and charging, the opposite happens. Lithium ions are released by the cathode and received by the anode. The amount of charge remaining in a device is essentially the concentration of ions on either side. For example, a fully charged device would see all the lithium ions concentrated on the anode side of battery.

In a lab condition, Ahmed would be able to disassemble the battery, separate the elements and see exactly how many lithium ions were on either side of the battery — giving him a precise measurement of the battery's lithium concentration and state of charge. But that's not feasible for real-world applications.

"When a lithium-ion battery is in use, you cannot dissemble it — you cannot measure it, “Ahmed explained.

To get an exact measurement, Ahmed places a small sensor on the outside of the battery to send an ultrasonic guided wave signal through the lithium-ion battery. The wave signal — which Ahmed views as data on his computer — allows him to more accurately judge the lithium concentration. If the wave signal is higher than the lithium concentration, the battery is charged. If lower, the battery is discharging. The rate at which the wave changes in relation to the lithium concentration provides an accurate measurement of the battery’s charge.

"By analyzing the wave signal's change, I can actually infer more accurately my state of charge and my state of health," Ahmed said.

This information can then be used to develop efficient algorithms to accurately predict the state of health and state of charge in lithium-ion batteries. Companies, especially those producing electric vehicles, would be interested in the accuracy of these algorithms, Ahmed noted.

Ahmed, who earned his doctorate from Rensselaer Polytechnic Institute in New York, began this work as a postdoctoral scholar at Stanford University. At Rensselaer, he focused his research on modeling ultrasonic guided waves and developing robust algorithm through those models. He then applied that expertise to lithium-ion batteries at Stanford.

"There was limited research on this specific area of lithium-ion batteries," Ahmed explained.

In the future, Ahmed will continue his work on lithium-ion batteries but is eager to expand his research into the growing field of robotics, where he sees a range of possibilities.