Nail penetration tests on a commercial battery (top) and a modified electrolyte battery (bottom)
Prof. I-Chun Lu, Chinese University of Hong Kong
Changing just one material used in lithium-ion batteries could prevent runaway fires that occur when they are punctured or bent, and mass production of these safer batteries could begin in the next few years.
Lithium-ion batteries used in smartphones, laptops and electric vehicles, have a graphite electrode, a metal oxide electrode and an electrolyte made of a lithium salt dissolved in a solvent. The liquid electrolyte allows ions to flow in one direction to charge the battery and in the other direction to release energy and power devices.
But if this structure is pierced in such a way that a short circuit occurs, all the chemical energy stored inside is quickly released, which can cause a fire or even an explosion.
Researchers have developed alternative battery designs to prevent such fires, including protective gels and even solid substitutes for liquid electrolyte. Now, Yue Song from the Chinese University of Hong Kong and her colleagues have created a safe design that can be built exactly like existing batteries by changing the electrolyte material.
Fires occur when negatively charged ions, called anions, break their bonds with the lithium in the battery. When bonds are broken, they release more heat and perpetuate 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, which binds to lithium from the existing solvent only at higher temperatures, when thermal runaway begins to occur. Unlike a conventional solvent, anionic bonds cannot exist in this new material and therefore cannot create a vicious cycle of heat generation. When pierced with a nail, the temperature inside the battery rose by only 3.5°C, while conventional batteries can heat up by more than 500°C.
“The bad guy is the anion, which has a lot of bond energy, and it's the breaking of those bonds that causes the thermal runaway effect,” says Gary Leak at the University of Birmingham, UK. “This isolates the bad guy from the process. It's a big leap forward in terms of battery safety.”
In tests, batteries using the new solvent retained 82 percent of their capacity over 4,100 hours of use, meaning they can compete with current technology.
Lick says the results could be used in the next generation of batteries, followed by mass production in three to five years.
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