Nail penetration tests on a commercial battery (top) and on one with a modified electrolyte (below)
Prof. Yi-Chun Lu, Chinese University of Hong Kong
Changing just one of the materials used in lithium-ion batteries could prevent the uncontrollable fires that erupt if they are pierced 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 cars have a graphite electrode, a metal oxide electrode and an electrolyte of 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 design is punctured in such a way that it creates a short circuit, all the chemical energy stored inside is released rapidly, 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 at the Chinese University of Hong Kong and her 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 lithium in the battery. As the bonds break, they release more heat and keep the destructive cycle going in a process called thermal runaway.
To get around this, the researchers created a second solvent called lithium bis(fluorosulfonyl)imide that bonds with the lithium from the existing solvent only at higher temperatures, when thermal runaway is beginning. Unlike the usual solvent, anion bonds can’t exist in this new material and therefore it can’t generate the vicious cycle of heat release. 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 boy is the anion, which has a lot of bond energy – and it’s these bonds breaking that causes thermal runaway,” says Gary Leeke at the University of Birmingham, UK. “It’s isolating the bad boy from that process. It’s a big leap in terms of battery safety.”
In tests, the batteries using the new solvent retained 82 per cent of their capacity over 4100 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 be mass-produced in three to five years.
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