Ultrasonic sensors as small as thumbnails can scan lithium-ion batteries to check their charge, health and safety, a new study has found.
The findings suggest that ultrasound, sound waves with frequencies higher than human hearing, may one day help electric vehicles better estimate the amount of charge left in a battery. The method may also help detect unstable batteries on the brink of disaster, quickly test battery quality during the manufacturing process and determine which used batteries are healthy enough to resell to reduce waste, said Hongbin Sun, the study’s lead author and an ultrasound engineer at Oak Ridge National Laboratory in Tennessee.
Estimating the remaining charge of commercial lithium-ion batteries is currently a challenging task. For example, electric vehicles typically experience about 10% uncertainty in estimating battery charge. This, in turn, reduces their driving range by about 10% to ensure they stay within the safe range of the battery.
To find a simple, inexpensive and non-destructive way to estimate the charge level of lithium-ion batteries, scientists have studied ultrasound. As lithium-ion batteries are discharged and recharged, underlying electrochemical reactions lead to mechanical changes in the performance and structure of their components, which in principle can be detected by ultrasonic scanning.
Previous work on ultrasonic scanning of lithium-ion batteries has typically used only one ultrasonic frequency for experiments. In this new study, the researchers examined multiple ultrasonic frequencies to understand the advantages and disadvantages of each in estimating the battery’s charge.
In the experiment, the scientists used a commercial off-the-shelf ultrasonic transducer about 1.25 centimeters wide to monitor a 2.4 amp-hour lithium-ion battery during charge and discharge. They tested relatively low-frequency 750 kHz, 1 MHz and 1.5 MHz ultrasonic waves, since the battery material absorbs to a large extent at frequencies higher than this. (For reference, humans can hear frequencies between about 20 hertz and 20 kHz.)
The researchers observed that the wave speeds at all three frequencies were related to the state of charge. The way the waves attenuate also helps reveal the phase changes of the materials in the battery during charging and discharging.
“Our proposed method provides a fast, low-cost solution for battery performance assessment and understanding material phase changes during battery charging and discharging,” Sun said, “For industrial-scale applications, it may be used to predict the state of charge of a battery and to assess whether the battery loses its ability to hold a charge. “
Sun noted that a potential application of this approach would be to embed low-cost piezoelectric ultrasonic sensors in electric vehicle batteries to continuously monitor them. The technology currently has an uncertainty of about 5 percent in estimating battery charge, about twice that of existing technology, he said. By improving sensor calibration and accounting for temperature effects during measurement, “we expect the error can be reduced to about 2 percent,” Sun said, “and we expect the reduction in uncertainty to translate into an improvement in EV range.”
In addition, using ultrasonic scanning, “defective cells can be easily detected,” Sun said. Ultrasonic scanning “may also provide early warning of battery fires, because ultrasound is very sensitive to temperature changes and material changes inside the battery.
Future research will need to test the technique on a wide range of lithium-ion batteries, as well as a range of charge rates and more charge cycles; Sun said the method “will require a lot of additional work and technical validation before it can be commercialized.