Tweaked lithium-ion battery can be pierced without catching fire
Nail penetration tests on a commercial battery (top) and one with a modified electrolyte (bottom)
Professor Yi-Chun Lu, Chinese University of Hong Kong
Changing just one of the materials used in lithium-ion batteries could prevent runaway fires that break out if they are punctured or bent, and mass production of these safer batteries could begin within the next few years.
Lithium-ion batteries used in smartphones, laptops and electric cars feature a graphite electrode, a metal oxide electrode and a lithium salt electrolyte dissolved in a solvent. The liquid electrolyte allows ions to flow in one direction to charge the battery and the other direction to release energy and power devices.
But if this design is pierced in such a way that it creates a short circuit, all the chemical energy stored inside is released quickly, which can cause a fire or even an explosion.
Researchers have developed alternative battery designs to prevent such fires, involving protective gels and even solid replacements for the liquid electrolyte. Now, Yue Sun of the Chinese University of Hong Kong and colleagues have created a safe design that can be built exactly like existing batteries, thanks to a change in the electrolyte material.
Fires occur when negatively charged ions, called anions, break their bonds with the lithium in the battery. As the bonds break, they release more heat and maintain the destructive cycle in a process called thermal runaway.
To get around this problem, the researchers created a second solvent called lithium bis(fluorosulfonyl)imide that binds to the existing solvent’s lithium only at higher temperatures, when thermal runaway begins. Unlike the usual solvent, anionic bonds cannot exist in this new material and therefore cannot generate the vicious cycle of heat release. When pierced with a nail, the temperature inside the battery only increases by 3.5°C, while conventional batteries can heat up by more than 500°C.
“The bad boy is the anion, which has a lot of bond energy – and it’s these bonds breaking that cause thermal runaway,” says Gary Leeke of the University of Birmingham, UK. “It isolates the bad boy from this process. It’s a big step forward in terms of battery safety.”
In testing, batteries using the new solvent retained 82% of their capacity over 4,100 hours of use, meaning they can compete with current technology.
Leeke says the findings could be incorporated into the next generation of batteries and then mass-produced within three to five years.
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