The sun was going down in Peoria when the fire crew arrived at the energy storage site in the Phoenix suburb. The 27 racks of batteries, set up by Arizona utility APS, had been charged up by solar panels on a day of scorching desert sun, and were ready to feed power back into the grid.

The fire department’s Hazmat unit had been dispatched to the site to investigate reports of smoke and a “nasty and acrid” smell coming from the facility. Shortly after the captain stepped inside the container housing the battery, a jet of flame shot out of it. He was thrust 20 metres into a chain-link fence, suffering brain and spinal injuries. Others suffered broken bones, a collapsed lung and nerve damage.

A year later, APS investigators claimed that a material build-up inside a battery cell had led to a chain event known as “thermal runaway” that had failed to be halted by a suppressant. Combustible gases had concentrated in the storage unit, creating a powder keg that was ignited when oxygen from the open door flooded in.

The explosion was shocking, in part because of its rarity. Incidents such as the 2019 Arizona failure, and a fire at a Tesla battery installation in Australia last week, are relatively uncommon, and technical improvements have only made the technology safer. But as large batteries are expected to feature more prominently, from home energy storage to electric cars, concerns over their safety are likely to become more prominent.

Safety fears

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Every incident, if rare, is seen as a setback for battery power. After the Arizona explosion, APS suspended battery installations, only resuming them last December, 19 months later.

Last month, General Motors was forced to recall its all-electric Chevy Bolts sold between 2017 and 2019, affecting around 69,000 vehicles, after two incidents in which the cars erupted into flames. It was the second recall of the vehicles over a potential battery fault in less than a year, and in both fire cases the vehicles had been treated in the previous recall.

GM told owners not to leave their vehicles unattended while charging overnight, and that as a precaution they should park them outside once they have been charged. It also urged owners not to charge their vehicles to full capacity. One of the owners whose cars had caught fire, a local politician called Timothy Briglin, had campaigned for greater electric vehicle adoption. “I have dozens of [constituents] who drive Bolts,” he told the website Electrek. “I’m worried for their safety.”

Building public confidence

Last week, a report from the House of Lords’ science and technology committee urged ministers to “take steps to build public confidence in the safety of batteries and hydrogen technologies”, saying that more stringent standards may be necessary.

Today’s lithium-ion battery cells are a finely tuned and coordinated piece of chemistry. Ions travel between two electrodes, giving off electrons as power. The electrodes are made of compounds designed to maximise capacity or power. Failure can occur when pushed to extremes, such as overcharging or overheating, or when metallic microstructures known as dendrites build up on an electrode.

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Jerry Barker, the chief scientist of British battery company Faradion, says that while the problem is limited in the most common lithium iron phosphate batteries, the nickel compounds that are prized in the electric car industry are more reactive.

“We all understand in the industry, although we don’t talk about it commercially too much, that if you go for the high energy density, you are compromising safety,” Barker says. His company is among several attempting to move on from lithium-ion batteries, which are dominant today, with new chemistries designed to be safer.

Lithium-ion batteries ‘incredibly safe’

Prof Paul Shearing, a battery expert at University College London’s Department of Chemical Engineering, says that although there is a public perception problem, lithium-ion batteries are “incredibly safe”.

“[These] are things that we use every single day. It’s extremely rare that any individual will be involved in any issue.”

Prof Shearing says that around one in 40m batteries goes wrong, a failure rate that compares well to conventional cars. “Every time there’s an electric vehicle fire it seems to be front page news. The fact is that there are petrol vehicle fires every single day up and down the country.”

However, he adds that safety could always be improved. “Clearly, we make billions of batteries a year and every year battery failure we would like to be able to avoid, particularly as we get into more mission critical applications.” Prof Shearing is involved in a £1.5m Government-funded project to better understand failures.

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Battery fires present other problems. They can be harder and more time-consuming to extinguish than traditional fires, and emit toxic fumes in the process. In April, a crashed Tesla Model S in Texas took eight firefighters seven hours to put out, in the process using 28,000 gallons of water – a volume the local fire department typically uses in a month.

Campaigners against battery facilities have pointed to the prospect of overwhelming fire departments.

One solar farm proposed by the energy firm Sunnica on the Cambridgeshire-Suffolk border, which would be among the largest in Britain, is attracting fierce opposition both for its proposed size, equivalent to 900 football pitches, and for the potential consequences of a battery failure. Edmund Fordham, of the group Say No To Sunnica, claims an explosion at the site could be several times bigger than the blast at the Port of Beirut last year. “If you really did have a major accident here, it would be utterly beyond the capacity of any local fire and rescue service to control it,” he says.

A spokesperson for the Sunnica project said it had consulted with local fire departments, adding: "Safety has been an important consideration in the design and preparation of Sunnica’s proposals and management plans."

Next-generation batteries

Almost by definition, any method of storing large amounts of energy carries risks. But the solution to concerns over batteries may be new chemistry. Barker’s Faradion is developing sodium-ion batteries which he says are less volatile, as well as using elements that are more widely available. “It’s a lot more robust from a safety angle,” he says.

Last week, Chinese battery manufacturing heavyweight CATL gave the technology a huge boost when it said it would start producing sodium-ion batteries for electric cars within two years.

Last week’s Lords’ report found that Britain has an opportunity to be a world leader in next-generation battery manufacturing in the next decade, and called for greater government support for the area. Fulfilling that promise might not only support Boris Johnson’s levelling up agenda, but safer batteries may also provide the reassurance that could be crucial to a net zero future.