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Prior to batteries depleting energy, failing abruptly, or igniting, they typically emit subtle sounds over time that reveal a profile of the degradation mechanisms occurring within their structure. However, until now, no one had deciphered the exact implications of those sounds or differentiated between normal background noise and critical indicators of potential issues.
Now, a group of scholars at MIT’s Department of Chemical Engineering has conducted an in-depth examination of the sounds produced by lithium-ion batteries and has successfully linked specific acoustic patterns to certain degradation mechanisms occurring within the cells. These recent discoveries may lay the groundwork for relatively straightforward, entirely passive, and non-invasive devices that could continuously assess the condition of battery systems, such as in electric vehicles or large-scale storage units, thereby facilitating predictions of operational longevity and anticipating failures before they transpire.
The results were published on Sept. 5 in the journal Joule, in a paper authored by MIT graduate students Yash Samantaray and Alexander Cohen, former MIT research scientist Daniel Cogswell PhD ’10, and Chevron Professor of Chemical Engineering and professor of mathematics Martin Z. Bazant.
“In this investigation, through precise scientific efforts, our team has been able to interpret the acoustic emissions,” Bazant states. “We could categorize them as originating from gas bubbles created by side reactions or by cracks from the expansion and contraction of the active material, and to identify signatures of those signals even amidst noisy data.”
Samantaray elaborates, “I believe the essence of this study is to explore methods for examining internal battery mechanisms while they are still charging and discharging, and to accomplish this non-invasively.” He continues, “Currently, there exist a few methods, but most are quite costly and not particularly suited for batteries in their conventional format.”
To perform their analysis, the team integrated electrochemical testing with the recording of acoustic emissions, under real-world charging and discharging scenarios, utilizing detailed signal processing to correlate the electrical and acoustic information. By doing so, he remarks, “we successfully developed a very affordable and effective method for understanding gas production and material fracture.”
Gas production and material cracking are two major mechanisms of degradation and failure in batteries, so the capability to recognize and differentiate those processes through sound monitoring could be an invaluable resource for those managing battery systems.
Earlier techniques simply observed sounds and recorded instances when the overall noise level surpassed a certain threshold. However, in this study, by concurrently monitoring the voltage and current alongside sound characteristics, Bazant mentions, “We understand that [sound] emissions occur at a specific potential [voltage], which assists us in identifying the process responsible for that emission.”
Following these evaluations, they would dismantle the batteries and examine them using an electron microscope to identify fractures in the materials.
Additionally, they implemented a wavelet transform—essentially, a means of encoding the frequency and length of each detected signal, offering unique signatures that are more easily extracted from background noise. “No one had previously achieved this,” Bazant asserts, “making it another significant advancement.”
Acoustic emissions are extensively utilized in engineering, he notes, for instance, to assess structures such as bridges for signs of impending failure. “It’s an excellent method for monitoring a system,” he states, “because those emissions occur regardless of whether you’re listening, so by paying attention, you can gain insights into internal processes that would otherwise remain hidden.”
With batteries, he remarks, “we often face challenges in interpreting voltage and current data as accurately as we desire to understand what’s happening within a cell. Therefore, this presents another perspective on the cell’s state of health, encompassing its residual useful life, as well as safety.” In a related paper with researchers from Oak Ridge National Laboratory, the team demonstrated that acoustic emissions can provide an early alert for thermal runaway, a condition that can result in fires if unaddressed. The recent study indicates that these sounds can be utilized to detect gas generation before ignition, “similar to observing the initial tiny bubbles in a pot of heated water long before it boils,” Bazant explains.
The next phase will involve applying this newfound understanding of how specific sounds correlate to particular conditions and developing a practical, affordable monitoring system based on these insights. For example, the team has secured funding from Tata Motors to create a battery monitoring system for its electric vehicles. “Now, we understand what to search for, and how to relate that to lifespan, health, and safety,” Bazant states.
One potential application of this new insight, Samantaray indicates, is “as a laboratory tool for teams developing new materials or testing new environments, enabling them to assess gas generation or active material fracturing without the need to disassemble the battery.”
Bazant adds that the system could also aid in quality assurance in battery manufacturing. “The most costly and time-consuming step in battery production is frequently the formation cycling,” he notes. This is the process where batteries are cycled through charging and discharging to break them in, and this includes chemical reactions that release some gas. The new system would facilitate the detection of these gas formation signatures, he explains, “and by identifying them, it may become easier to distinguish well-formed cells from poorly formed ones very early in the production process, even before the battery’s useful life begins.”
The effort received support from the Toyota Research Institute, the Center for Battery Sustainability, the National Science Foundation, and the Department of Defense, while utilizing the facilities of MIT.nano.
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