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A savage fire erupted in an e-bike repair shop in New York City in June. Four people died. It was just one of hundreds of fires caused by electric scooters and e-bikes that now blaze through New York and other cities each year, claiming lives and destroying buildings. When the lithium-ion batteries that power these bikes and scooters are poorly made, overused, or charged for too long, they can violently explode and cause deadly blazes that are hard to extinguish.

Compared with older lead-acid and nickel-cadmium batteries, lithium-ion batteries charge faster, last longer, and pack more power into a smaller package. They revolutionized communications and transportation, enabling pocket-sized cell phones and long-range electric cars. They’re now widely used in smartphones, laptop computers, cameras, and power tools.

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Most of today’s modern lithium batteries operate safely, says New York University mechanical engineer Nikhil Gupta. But some of them are cheaply made and catch fire, like the one in the New York City e-bike shop. Lithium-battery chemistry differs from that of most other common batteries because it uses a flammable organic electrolyte to help generate a current, part of what creates a fire risk if the battery gets too hot.​​

Each battery has a “temperature of no return.”

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Lithium-ion batteries consist of a protective circuit board and a single or multiple cells, each containing an anode, cathode, separator, and electrolyte; the anode and cathode—typically graphite and cobalt oxide, respectively—are sandwiched between conductive foils connected to the terminals of the cell, which transmit current between the battery, the device, and the energy source that powers the battery.

When they’re operating normally, lithium batteries create a current via the movement of lithium ions swarming from the anode to the cathode, across a permeable separator that holds electrolyte. The electrons can’t cross the electrolyte, so they’re forced through a current collector. While charging, the battery’s lithium atoms move in the other direction, from the cathode to the anode, where they are ionized—or separated from their electrons.

Most lithium battery fires result from an internal short circuit, which can occur when the layers of a battery cell aren’t perfectly aligned. A bike crash could shift one piece out of place. A dust particle introduced during manufacturing could corrode the layers over time. Or too high a voltage could create intense heat that causes the materials to expand or collapse. Even a slip of the separator allows the electrons to pass through the battery cell’s interior. That’s when everything starts going awry.

The electrons will flock toward this new path, rather than traveling in a circuit outside it. “They’re funny things,” says Anna Stefanopoulou, a University of Michigan mechanical engineer. “They always find the easiest path of least resistance.” All these electrons pack into a tight spot, and it gets hot very quickly.

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The battery chemistry that leads to a fire varies, Gupta says, and we don’t know if there are specific materials used in the batteries which are more dangerous than others. But each battery has a “temperature of no return.” Once it’s reached, the battery won’t be able to cool itself down. Battery components start to melt and decompose, generating flammable gas, such as hydrogen gas. Combined with oxygen from the environment and battery materials that act as fuel, the cell catches fire.

Unless there’s a robust cooling system to stop them, or a sensor that detects gases and shuts the battery down, the fire spreads to other battery cells. Many batteries are designed to include these safety sensors and coolers, but they tend to add weight, size, and cost to its manufacture. This trade-off means batteries designed to be cheap and compact, like those for e-bikes, may lack these failsafes, Stefanopoulou said.

Extinguishing a lithium-ion battery fire is difficult. “The first instinct would be to get water and throw it on this fire, but that fire is just going to be worse because of the water,” Gupta says. You need a special kind of foam to squelch this kind of fire, he adds, and most people aren’t going to have that handy. The best you can do is keep and charge your battery in a cool place.

Lead image by Tasnuva Elahi; with images by Markus Mainka and YummyBuum / Shutterstock

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