Kyle Proffitt
February 24, 2025 | When the rechargeable batteries in your portable electronic device begin to die, especially when this seems to happen prematurely (probably right after the warranty expires), it makes sense to wonder why. Most of us would be left frustrated with little means of further understanding. However, when Dr. Yijin Liu, Associate Professor of Mechanical Engineering at UT Austin, experienced a failing earbud, he saw opportunity instead of despair. Liu is a battery researcher, but much of this work involves studies on isolated batteries. “Our laboratory battery tests need to better represent how these batteries are used in the real world, and that’s what we’re trying to do,” Liu explained.
With international connections at billion-dollar x-ray facilities, he decided to scrutinize the failure mechanism at the greatest level of detail. The result is a detailed depiction of how battery and other device components interact in a small consumer electronic device to create temperature-dependent gradients and potential early failure.
The research effort, published last month in Advanced Materials (DOI: 10.1002/adma.202416915), saw Liu teamed up with another 20 authors from 7 different institutions. “We utilized experimental capabilities at UT Austin (Texas), Stanford Linear Accelerator (SLAC) National Lab (California), Brookhaven National Lab (New York), Argonne National Lab (Illinois), and European Synchrotron Radiation Facility (ESRF) (France),” Liu explained to Battery Power Online by email. Purdue University and Sigray, Inc. rounded out the institutions involved. A major team effort with extensive planning and coordination was necessary to reveal “the battery’s ‘secret life’ within this compact electronic device,” Liu said.
A Look Inside
At one level, the cause of Liu’s failing earbud was trivial—he primarily used just the one. However, having its twin in near-original condition made for a nice experimental setup to compare and explore the underlying mechanism. His group started with electrochemical experiments, showing that the failing earbud held less than half the capacity of the other. The bad earbud also showed greater impedance (electrons and lithium ions don’t move as easily through the battery) and self-discharge, rendering it pretty useless for regular use.
From here, the earbuds were subjected to X-ray tomography using in-house laboratory equipment, which non-destructively revealed the internal components. Inside the earbud, they could see that the in-ear head portion contains a speaker, sensors, and printed circuit boards, whereas the external “stem” portion primarily houses a cylindrical battery with a Bluetooth antenna wrapped around it. Charging contacts are at the base of the battery. The researchers looked at the earbuds both in and out of their charging case, which contains its own pouch cell battery for recharging on the go. Zooming in on the jelly-roll structure of the cylindrical cell, down to about 0.3 microns of resolution in the faulty earbud, they could see deformation near the positive terminal at the base. They saw several other signs of degradation too, including electrolyte depletion, metal precipitation, electrode corrosion, and current collector deformation. The degradation seen at this level of detail was worse primarily toward the bottom of the battery.
Traveling Earbuds
Both earbuds were then disassembled, and the cathode and anode sheets were unrolled. Macroscopically, anode exfoliation and adhesion were apparent at the top and bottom of the roll in the faulty cell. Coin cells were created from punched discs of cathode material, revealing lost capacity along the entire battery, but greatest loss was at the top and bottom. From here, cathode material samples were sent on a national and international tour of different high-energy synchrotron-based X-ray facilities. Liu explained how each facility contributed to a deeper understanding of the chemical makeup, oxidation state, and crystal structure arrangements:
“The transmission X-ray microscopy beamline at Stanford Synchrotron Radiation Lightsource revealed the chemical heterogeneity within individual particles (a few micrometers in size). The hard X-ray nanoprobe beamline at Brookhaven National Laboratory probes local phase transition at nanoscale, which is a key indicator of the material degradation. The hard X-ray spectroscopy beamline at Argonne National Laboratory helps us to understand the atomic bonding and its evolution as the battery ages. The ESRF in France allows us to look at thousands of particles, offering statistical insights.”
The result from these varied methods was a very detailed picture, down to individual LiCoO2 (LCO) cathode particles. The researchers point out that LCO is particularly amenable to the X-ray methods applied. They found that at the cathode surface, the cobalt was more reduced, or in a lower oxidation state, at the top and bottom of the battery. They attribute this cobalt reduction to side reactions, which increase impedance in these regions. However, bulk-level analysis with hard X-rays that reach deeper into the cathode revealed a higher cobalt valence (more oxidized) across the cell relative to the control earbud. This result indicates a loss of active lithium, meaning the battery can not fully discharge. Additional X-ray diffraction studies showed undesirable changes in the cathode crystal structure, and these changes were again worst at the bottom of the battery with additional damage at the top.
The sum of these experiments was a revelation that battery degradation occurred non-homogeneously. It was worst at the bottom, but it improved toward the middle before worsening again at the top. The researchers reasoned that the battery’s design would direct current toward the bottom, where the charging tabs are located, and explain heightened degradation there, but additional degradation at the top of the battery was unexpected. They turned to thermal imaging to see what happens during charging.
Alternating Gradients
As anticipated, charging an isolated earbud battery (from new earbuds, since the old ones were destroyed) showed a temperature gradient with more heat at the bottom. However, charging the earbud in its case produced the opposite outcome; most of the heat is up top, with a significant peak appearing in the in-ear portion, external to the battery. Additional experiments suggested that this heat originated in the earbud itself, but not from the battery. “The battery is influenced by other device components, such as the printed circuit and microphone,” Liu explained. “These components generate heat, creating a temperature gradient that impacts battery degradation.” There were actually two competing temperature gradients, one intrinsic to the battery itself, and one external, related to the other earbud components and charging environment. The top-heavy heat gradient produced while charging predominated, but degradation occurred at both ends. Liu also pointed out that they confirmed results with additional earbuds. “We purposely purchased secondhand earbuds of the same model from eBay; the pattern is consistent,” he said.
Academia Meets the Real World
Liu explained why these experiments are important. Researchers are used to testing batteries in a range of temperature conditions to see how they’ll perform in extreme environments. However, “we often do so by using an oven, which creates a uniform and stable temperature profile; now we know that this is overly simplified,” Liu said. Additionally, “real-world applications require battery integration into larger systems; this system-level packaging creates a microenvironment that influences cell performance, highlighting a key gap between academic research and real-world application.” The report adds that “mild and intermediate temperature effects are not well understood and, frankly, often overlooked.”
Asked about whether this damage could translate to bigger problems, Liu said that “it is also important to look at this from the safety perspective. Is the decayed cell more prone to thermal runaway? This is a very important research topic.” He added that for second-life battery applications, such as retired EV packs in household energy storage, “this needs to be evaluated very carefully.”
Changes Ahead
Industry moves fast. If you buy a newer version of the same earbuds, “they are already manufactured with a different cell configuration and a different packaging design,” Liu said. However, he explained that “in academia, we are going after an in-depth understanding.”
He believes there is a mutually beneficial partnership available that can lead to improved design based on better fundamental understanding. “We strongly believe that a closer interaction between academia and industry will be very valuable for both parties,” he said. He continued, explaining the offer. “We are using advanced characterization techniques to pinpoint the material level evolution, which could offer insights that are not previously available.”
The work also suggests that temperature sensors need to be incorporated for certain designs. The report states that “to truly enable battery temperature monitoring in real-life applications, future developments and implementation of low-cost and compact temperature sensors are needed.” Liu explained that this could show up in larger packs like those for EVs, which have more degrees of freedom for design. “If there is an unavoidable temperature gradient for some reason, one could think of a cooling system with a gradient to mitigate.” In one sign of what’s ahead, they are now studying how charging protocols for different brands or models of EVs affect batteries. “We noticed that these charging protocols are very different, and we are trying to understand the design considerations and their behaviors and impacts,” Liu said.
