In today’s electronic age, rechargeable lithium-ion batteries are ubiquitous. Compared with the lead-acid versions that have dominated the battery market for decades, lithium-ion batteries can charge faster and store more energy for the same amount of weight.

These devices make our electronic gadgets and electric cars lighter and longer-lasting – but they also have disadvantages. They contain a lot of energy, and if they catch fire, they burn until all of that stored energy is released. A sudden release of huge amounts of energy can lead to explosions that threaten lives and property.

As scientists who study energy generation, storage and conversion, and automotive engineering, we have a strong interest in the development of batteries that are energy-dense and safe. And we see encouraging signs that battery manufacturers are making progress toward solving this significant technical problem.

Urban transportation is undergoing a transformative shift toward electrification. As concerns grow in cities around the world about climate change and air quality, electric vehicles have taken center stage.

At the same time, e-bikes and electric scooters are transforming urban transit by providing convenient, low-carbon ways to navigate crowded streets and reduce traffic congestion. From 2010 through 2022, shared e-bikes and e-scooters – those owned by rental networks – accounted for more than half a billion trips in U.S. cities. Privately owned e-bikes add to that total: In 2021, more than 880,000 e-bikes were sold in the U.S., compared with 608,000 electric cars and trucks.

Battery-powered vehicles account for a small share of car fires, but controlling EV fires is difficult. Typically, an EV fire burns at roughly 5,000 degrees Fahrenheit (2,760 Celsius), while a gasoline-powered vehicle on fire burns at 1,500 F (815 C). It takes about 2,000 gallons of water to extinguish a burning gasoline-powered vehicle; putting out an EV fire can take 10 times more.

This is a major concern in large cities where electric vehicles are popular. Fire departments in New York City and San Francisco report handling more than 660 fires involving lithium-ion batteries since 2019. In New York City, these fires caused 12 deaths and more than 260 injuries from 2021 through early 2023. Clearly, there is a need for safer handling and charging practices, as well as technical improvements to batteries.

To understand lithium-ion battery fires, it’s important to know some basics. A battery holds chemicals that contain energy, with a separator between its positive and negative electrodes. It works by converting this energy into electricity.

The two electrodes in a battery are surrounded by an electrolyte – a substance that allows an electrical charge to flow between the two terminals. In a lithium-ion battery, for example, lithium ions carry the electric charge. When a device is connected to a battery, chemical reactions take place on the electrodes and create a flow of electrons in the external circuit that powers the device.

Cellphones and digital cameras can operate on a single battery, but an electric car needs much more energy and power. Depending on its design, an EV may contain dozens to thousands of single batteries, which are known as cells. Cells are clustered together in sets called modules, which in turn are assembled together in packs. A standard EV will contain one large battery pack with many cells inside it.

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