A new polymer-based electrolyte made Li-ion batteries non-flammable

"One of the biggest challenges in the battery industry is this safety issue, so there's a lot of effort going into trying to make a battery electrolyte that is safe," said Rachel Z Huang, a graduate student at Stanford University and first author of the new study.

An extra salty electrolyte

Conventional lithium-ion battery electrolytes are made of lithium salt dissolved in a liquid organic solvent, such as ether or carbonate. While this solvent improves battery performance by helping to move lithium ions around, it's also a potential firestarter.

At temperatures above 140 degrees Fahrenheit (60 degrees Celsius), the solvent used in conventional electrolytes starts to evaporate, transforming from liquid to gas, and causing the battery to swell up like a balloon. The gas then catches fire, causing the entire battery to explode.

Researchers have been trying to develop non-flammable electrolytes for decades now. This includes polymer electrolytes, which use a polymer matrix instead of the classic salt-solvent solution to move ions around. However, these alternative electrolytes underperform in comparison to conventional ones.

The team led by Yi Cui, a professor at SLAC National Accelerator Laboratory and Stanford, wanted to produce a polymer-based electrolyte that could offer both safety and performance.

And the trick that made the electrolyte safe and efficient was adding as much of a particular lithium salt (LiFSI) as possible to a polymer-based electrolyte, bumping the mix from less than half of the electrolyte's weight to 63 percent. The additional salt anchors the molecules of the electrolyte, preventing them from evaporating in high temperatures and thus stopping them from catching fire. In temperature tests, the lithium-ion battery continued functioning at temperatures as high as 212 degrees Fahrenheit (100 degrees Celsius).

"I just wanted to see how much I could add and test the limit," Huang said. Usually, less than 50% of a polymer-based electrolyte's weight is salt. Huang bumped that number to 63 percent, creating one of the saltiest polymer-based electrolytes ever.

Gooey flameproof batteries

One key feature of the team's new electrolyte is that it has a gooey form similar to conventional electrolytes, which means it can be integrated with existing battery parts, unlike other experimental, non-flammable electrolytes.

The research team put forward that this could be used in electric cars. If the lithium-ion batteries in EVs were packed with this new electrolyte, then the risk of combustion would be much lower. It could also lead to more efficiency in batteries, with less space dedicated to cooling systems and more dedicated to battery capacity.

"This very exciting new battery electrolyte is compatible with the existing lithium ion-battery cell technology and would make big impacts on consumer electronics and electrical transportation," said Professor Yi Cui.

The study was published in the journal Matter.