Batteries to firebombs

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By 2017-03-19

By Chelsea Leu

Who runs the world? Lithium-ion batteries! (Sorry, Beyoncé.) Ever since Sony commercialized the chemistry in 1991, Li-ion cells have powered everything from the Mars Curiosity rover to the device closest to you. The tech has endured for good reason: It charges quickly, fits a ton of energy into a slim package (lithium is the lightest metal and is highly reactive), and is generally pretty safe. But when things go wrong, they go very wrong. Scores of Galaxy Note 7s and hoverboards have succumbed to the fiery embrace of a malfunctioning Li-ion battery. Here is what's inside the little fuel packs that power your life – and how they can turn into battery flambé.

Lithium Cobalt Oxide

To store or release energy in a slim, efficient Li-ion power pack, lithium ions ping-pong between two electrodes: a sheet of lithium cobalt oxide and a sheet of graphite. When you charge your new drone (or smartphone or laptop or hoverboard), electrons flowing in from the outlet help lure lithium ions out of the LiCoO2, which then migrate to the graphite electrode and wait to be released – along with electrons (energy!) – later. Within the Li electrode, cobalt and oxygen form sturdy layers of octa­hedrons, which keep the molecule from collapsing as ions enter and exit. But at high temperatures that edifice can crumble, contributing to a very combustible situation.

Graphite

A mineral form of pure carbon, best known as the writey part in pencils. Graphite forms the second electrode, and the lithium ions lodge within it as the battery charges. Then, when you turn on your MacBook, those ions get pulled out of the graphite to journey back to the lithium cobalt oxide, a process that produces the electricity to let you browse Facebook – we mean respond to all those work emails.

Polypropylene

A thin slab of this plastic keeps the electrodes apart. The separator, as it's called, is perforated with micron-scale holes to let Li-ions pass through. It's often to blame if your phone becomes a smoking inferno – faulty separators can let the electrodes touch, triggering a process known as thermal runaway. That can quickly generate huge amounts of heat (up to 926.7 degrees Celsius) and pressure, causing the flammable substances present to burst into flames.

Ethylene Carbonate

This clear, flammable, organic solvent helps shuttle ions back and forth. If a malfunctioning battery heats up enough (say, from a bad separator), the hot liquid can escape the case, react with oxygen in air, and kaboom!

Lithium Hexa­fluorophosphate

A white powdery substance that's dissolved into the ethylene carbonate, LiPF6 just sort of floats around in the battery, supplying additional lithium ions to speed up charging and discharging. This compound is not flammable. Yay! But it does burn skin on contact. Boo!

(Source: Wired.com)

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