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.

Essay 106: World Watching: Project Syndicate—New Commentary

from Project Syndicate:

The EU’s EV Greenwash

by Hans-Werner Sinn

EU emissions regulations that went into force earlier this year are clearly designed to push diesel and other internal-combustion-engine automobiles out of the European market to make way for electric vehicles. But are EVs really as climate-friendly and effective as their promoters claim?

MUNICHGermany’s automobile industry is its most important industrial sector. But it is in crisis, and not only because it is suffering the effects of a recession brought on by Volkswagen’s own cheating on emissions standards, which sent consumers elsewhere. The sector is also facing the existential threat of exceedingly strict European Union emissions requirements, which are only seemingly grounded in environmental policy.

The EU clearly overstepped the mark with the carbon dioxide regulation [PDF] that went into effect on April 17, 2019. From 2030 onward, European carmakers must have achieved average vehicle emissions of just 59 grams of CO2 per kilometer, which corresponds to fuel consumption of 2.2 liters of diesel equivalent per 100 kilometers (107 miles per gallon). This simply will not be possible.

As late as 2006, average emissions for new passenger vehicles registered in the EU were around 161 g/km. As cars became smaller and lighter, that figure fell to 118 g/km in 2016. But this average crept back up, owing to an increase in the market share of gasoline engines, which emit more CO2 than diesel engines do. By 2018, the average emissions of newly registered cars had once again climbed to slightly above 120 g/km, which is twice what will be permitted in the long term.

Even the most gifted engineers will not be able to build internal combustion engines (ICEs) that meet the EU’s prescribed standards (unless they force their customers into soapbox cars). But, apparently, that is precisely the point. The EU wants to reduce fleet emissions by forcing a shift to electric vehicles. After all, in its legally binding formula for calculating fleet emissions, it simply assumes that EVs do not emit any CO2 whatsoever.

The implication is that if an auto company’s production is split evenly between EVs and ICE vehicles that conform to the present average, the 59 g/km target will be just within reach. If a company cannot produce EVs and remains at the current average emissions level, it will have to pay a fine of around €6,000 ($6,600) per car, or otherwise merge with a competitor that can build EVs.

But the EU’s formula is nothing but a huge scam. EVs also emit substantial amounts of CO2, the only difference being that the exhaust is released at a remove—that is, at the power plant. As long as coal– or gas-fired power plants are needed to ensure energy supply during the “dark doldrums” when the wind is not blowing and the sun is not shining, EVs, like ICE vehicles, run partly on hydrocarbons. And even when they are charged with solar– or wind-generated energy, enormous amounts of fossil fuels are used to produce EV batteries in China and elsewhere, offsetting the supposed emissions reduction. As such, the EU’s intervention is not much better than a cut-off device for an emissions control system.

Earlier this year, the physicist Christoph Buchal and I published a research paper [PDF, in German] showing that, in the context of Germany’s energy mix, an EV emits a bit more CO2 than a modern diesel car, even though its battery offers drivers barely more than half the range of a tank of diesel. And shortly thereafter, data published [PDF, in German] by Volkswagen confirmed that its e-Rabbit vehicle emits slightly more CO2 [PDF, in German] than its Rabbit Diesel within the German energy mix. (When based on the overall European energy mix, which includes a huge share of nuclear energy from France, the e-Rabbit fares slightly better than the Rabbit Diesel.)

Adding further evidence, the Austrian think tank Joanneum Research has just published a large-scale study [PDF, in German] commissioned by the Austrian automobile association, ÖAMTC, and its German counterpart, ADAC, that also confirms those findings. According to this study, a mid-sized electric passenger car in Germany must drive 219,000 kilometers before it starts outperforming the corresponding diesel car in terms of CO2 emissions. The problem, of course, is that passenger cars in Europe last for only 180,000 kilometers, on average. Worse, according to Joanneum, EV batteries don’t last long enough to achieve that distance in the first place. Unfortunately, drivers’ anxiety about the cars’ range prompts them to recharge their batteries too often, at every opportunity, and at a high speed, which is bad for durability.

As for EU lawmakers, there are now only two explanations for what is going on: either they didn’t know what they were doing, or they deliberately took Europeans for a ride. Both scenarios suggest that the EU should reverse its interventionist industrial policy, and instead rely on market-based instruments such as a comprehensive emissions trading system.

With Germany’s energy mix, the EU’s regulation on fleet fuel consumption will not do anything to protect the climate. It will, however, destroy jobs, sap growth, and increase the public’s distrust in the EU’s increasingly opaque bureaucracy.