Science-Watching: Why Do Batteries Sometimes Catch Fire and Explode?

[from Berkeley Lab News, by Theresa Duque]

Key Takeaways
  • Scientists have gained new insight into why thermal runaway, while rare, could cause a resting battery to overheat and catch fire.
  • In order to better understand how a resting battery might undergo thermal runaway after fast charging, scientists are using a technique called “operando X-ray microtomography” to measure changes in the state of charge at the particle level inside a lithium-ion battery after it’s been charged.
  • Their work shows for the first time that it is possible to directly measure current inside a resting battery even when the external current measurement is zero.
  • Much more work is needed before the findings can be used to develop improved safety protocols.

How likely would an electric vehicle battery self-combust and explode? The chances of that happening are actually pretty slim: Some analysts say that gasoline vehicles are nearly 30 times more likely to catch fire than electric vehicles. But recent news of EVs catching fire while parked have left many consumers – and researchers – scratching their heads over how these rare events could possibly happen.

Researchers have long known that high electric currents can lead to “thermal runaway” – a chain reaction that can cause a battery to overheat, catch fire, and explode. But without a reliable method to measure currents inside a resting battery, it has not been clear why some batteries go into thermal runaway, even when an EV is parked.

Now, by using an imaging technique called “operando X-ray microtomography,” scientists at Lawrence Berkeley National Laboratory (Berkeley Lab) and UC Berkeley have shown that the presence of large local currents inside batteries at rest after fast charging could be one of the causes behind thermal runaway. Their findings were reported in the journal ACS Nano.

“We are the first to capture real-time 3D images that measure changes in the state of charge at the particle level inside a lithium-ion battery after it’s been charged,” said Nitash P. Balsara, the senior author on the study. Balsara is a faculty senior scientist in Berkeley Lab’s Materials Sciences Division and a UC Berkeley professor of chemical and biomolecular engineering.

“What’s exciting about this work is that Nitash Balsara’s group isn’t just looking at images – They’re using the images to determine how batteries work and change in a time-dependent way. This study is a culmination of many years of work,” said co-author Dilworth Y. Parkinson, staff scientist and deputy for photon science operations at Berkeley Lab’s Advanced Light Source (ALS).

The team is also the first to measure ionic currents at the particle level inside the battery electrode.

3D microtomography experiments at the Advanced Light Source enabled researchers to pinpoint which particles generated current densities as high as 25 milliamps per centimeter squared inside a resting battery after fast charging. In comparison, the current density required to charge the test battery in 10 minutes was 18 milliamps per centimeter squared. (Credit: Nitash Balsara and Alec S. Ho/Berkeley Lab. Courtesy of ACS Nano)
Measuring a battery’s internal currents

In a lithium-ion battery, the anode component of the electrode is mostly made of graphite. When a healthy battery is charged slowly, lithium ions weave themselves between the layers of graphite sheets in the electrode. In contrast, when the battery is charged rapidly, the lithium ions have a tendency to deposit on the surface of the graphite particles in the form of lithium metal.

“What happens after fast charging when the battery is at rest is a little mysterious,” Balsara said. But the method used for the new study revealed important clues.

Experiments led by first author Alec S. Ho at the ALS show that when graphite is “fully lithiated” or fully charged, it expands a tiny bit, about a 10% change in volume – and that current in the battery at the particle level could be determined by tracking the local lithiation in the electrode. (Ho recently completed his Ph.D. in the Balsara group at UC Berkeley.)

A conventional voltmeter would tell you that when a battery is turned off, and disconnected from both the charging station and the electric motor, the overall current in the battery is zero.

But in the new study, the research team found that after charging the battery in 10 minutes, the local currents in a battery at rest (or currents inside the battery at the particle level) were surprisingly large. Parkinson’s 3D microtomography instrument at the ALS enabled the researchers to pinpoint which particles inside the battery were the “outliers” generating alarming current densities as high as 25 milliamps per centimeter squared. In comparison, the current density required to charge the battery in 10 minutes was 18 milliamps per centimeter squared.

The researchers also learned that the measured internal currents decreased substantially in about 20 minutes. Much more work is needed before their approach can be used to develop improved safety protocols.

Researchers from Argonne National Laboratory also contributed to the work.

The Advanced Light Source is a DOE Office of Science user facility at Berkeley Lab.

The work was supported by the Department of Energy’s Office of Science and Office of Energy Efficiency and Renewable Energy. Additional funding was provided by the National Science Foundation.

New Ultrathin Capacitor Could Enable Energy-Efficient Microchips

Scientists turn century-old material into a thin film for next-gen memory and logic devices

[from Berkeley Lab, by Rachel Berkowitz]

Electron microscope images show the precise atom-by-atom structure of a barium titanate (BaTiO3) thin film sandwiched between layers of strontium ruthenate (SrRuO3) metal to make a tiny capacitor. (Credit: Lane Martin/Berkeley Lab)

The silicon-based computer chips that power our modern devices require vast amounts of energy to operate. Despite ever-improving computing efficiency, information technology is projected to consume around 25% of all primary energy produced by 2030. Researchers in the microelectronics and materials sciences communities are seeking ways to sustainably manage the global need for computing power.

The holy grail for reducing this digital demand is to develop microelectronics that operate at much lower voltages, which would require less energy and is a primary goal of efforts to move beyond today’s state-of-the-art CMOS (complementary metaloxide semiconductor) devices.

Non-silicon materials with enticing properties for memory and logic devices exist; but their common bulk form still requires large voltages to manipulate, making them incompatible with modern electronics. Designing thin-film alternatives that not only perform well at low operating voltages but can also be packed into microelectronic devices remains a challenge.

Now, a team of researchers at Lawrence Berkeley National Laboratory (Berkeley Lab) and UC Berkeley have identified one energy-efficient route—by synthesizing a thin-layer version of a well-known material whose properties are exactly what’s needed for next-generation devices.

First discovered more than 80 years ago, barium titanate (BaTiO3) found use in various capacitors for electronic circuits, ultrasonic generators, transducers, and even sonar.

Crystals of the material respond quickly to a small electric field, flip-flopping the orientation of the charged atoms that make up the material in a reversible but permanent manner even if the applied field is removed. This provides a way to switch between the proverbial “0” and “1” states in logic and memory storage devices—but still requires voltages larger than 1,000 millivolts (mV) for doing so.

Seeking to harness these properties for use in microchips, the Berkeley Lab-led team developed a pathway for creating films of BaTiO3 just 25 nanometers thin—less than a thousandth of a human hair’s width—whose orientation of charged atoms, or polarization, switches as quickly and efficiently as in the bulk version.

“We’ve known about BaTiO3 for the better part of a century and we’ve known how to make thin films of this material for over 40 years. But until now, nobody could make a film that could get close to the structure or performance that could be achieved in bulk,” said Lane Martin, a faculty scientist in the Materials Sciences Division (MSD) at Berkeley Lab and professor of materials science and engineering at UC Berkeley who led the work.

Historically, synthesis attempts have resulted in films that contain higher concentrations of “defects”—points where the structure differs from an idealized version of the material—as compared to bulk versions. Such a high concentration of defects negatively impacts the performance of thin films. Martin and colleagues developed an approach to growing the films that limits those defects. The findings were published in the journal Nature Materials.

To understand what it takes to produce the best, low-defect BaTiO3 thin films, the researchers turned to a process called pulsed-laser deposition. Firing a powerful beam of an ultraviolet laser light onto a ceramic target of BaTiO3 causes the material to transform into a plasma, which then transmits atoms from the target onto a surface to grow the film. “It’s a versatile tool where we can tweak a lot of knobs in the film’s growth and see which are most important for controlling the properties,” said Martin.

Martin and his colleagues showed that their method could achieve precise control over the deposited film’s structure, chemistry, thickness, and interfaces with metal electrodes. By chopping each deposited sample in half and looking at its structure atom by atom using tools at the National Center for Electron Microscopy at Berkeley Lab’s Molecular Foundry, the researchers revealed a version that precisely mimicked an extremely thin slice of the bulk.

“It’s fun to think that we can take these classic materials that we thought we knew everything about, and flip them on their head with new approaches to making and characterizing them,” said Martin.

Finally, by placing a film of BaTiO3 in between two metal layers, Martin and his team created tiny capacitors—the electronic components that rapidly store and release energy in a circuit. Applying voltages of 100 mV or less and measuring the current that emerges showed that the film’s polarization switched within two billionths of a second and could potentially be faster—competitive with what it takes for today’s computers to access memory or perform calculations.

The work follows the bigger goal of creating materials with small switching voltages, and examining how interfaces with the metal components necessary for devices impact such materials. “This is a good early victory in our pursuit of low-power electronics that go beyond what is possible with silicon-based electronics today,” said Martin.

“Unlike our new devices, the capacitors used in chips today don’t hold their data unless you keep applying a voltage,” said Martin. And current technologies generally work at 500 to 600 mV, while a thin film version could work at 50 to 100 mV or less. Together, these measurements demonstrate a successful optimization of voltage and polarization robustness—which tend to be a trade-off, especially in thin materials.

Next, the team plans to shrink the material down even thinner to make it compatible with real devices in computers and study how it behaves at those tiny dimensions. At the same time, they will work with collaborators at companies such as Intel Corp. to test the feasibility in first-generation electronic devices. “If you could make each logic operation in a computer a million times more efficient, think how much energy you save. That’s why we’re doing this,” said Martin.

This research was supported by the U.S. Department of Energy (DOE) Office of Science. The Molecular Foundry is a DOE Office of Science user facility at Berkeley Lab.