Flameproofing Lithium-Ion Batteries with Salt

“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%, creating one of the saltiest polymer-based electrolytes ever.

Unlike other polymer-based electrolytes, this one also contained flammable solvent molecules. However, the overall electrolyte, known as Solvent-Anchored non-Flammable Electrolyte (SAFE), proved non-flammable at high temperatures during tests in a lithium-ion battery.

SAFE works because the solvents and salt work together. The solvent molecules help conduct ions, resulting in performance comparable to that of batteries containing conventional electrolytes. But, instead of failing at high temperatures like most lithium-ion batteries, batteries containing SAFE continue to operate at temperatures between 77–212 degrees F.

Meanwhile, the ample added salts act as anchors for the solvent molecules, preventing them from evaporating and catching fire.

“This new finding points out a new way of thinking for polymer-based electrolyte design,” said Zhenan Bao, a professor at Stanford University and investigator with the Stanford Institute for Materials and Energy Sciences (SIMES) who advises Huang. “This electrolyte is important for developing future batteries that are both high energy density and safe.”

Staying Gooey
Polymer-based electrolytes can be solid or liquid. Importantly, the solvents and salt in SAFE plasticize its polymer matrix to make it a goo-like liquid, just like conventional electrolytes. 

One benefit: A gooey electrolyte can fit into existing, commercially available lithium-ion battery parts, unlike other non-flammable electrolytes that have emerged. Solid-state ceramic electrolytes, for example, must use specially designed electrodes, making them costly to produce.

“With SAFE there’s no need to change any of the manufacturing setup,” Huang said. “Of course, if it is ever used for production there are optimizations needed for the electrolyte to fit into the production line, but the work is a lot less than any of the other systems.”

Yi Cui, a professor at SLAC and Stanford and a SIMES investigator who also advises Huang, said, “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.”

One application of SAFE may be in electric cars. 

If the multiple lithium-ion batteries in an electric car sit too close together, they can heat each other up, which could eventually lead to overheating and fire. But, if an electric car contains batteries filled with an electrolyte like SAFE that is stable at high temperatures, its batteries can be packed close together without worry of overheating. 

In addition to mitigating fire risk, this means less space occupied by cooling systems and more space for batteries. More batteries increase the overall energy density, meaning the car could go longer between charging. 

“So it’s not just a safety benefit,” said Huang. “This electrolyte could also allow you to pack in a lot more batteries.”

Time will tell what other battery-powered products could get a little SAFE-er. 

Chris Patrick is a freelance writer. The article was originally posted to the website of the Stanford Linear Accelerator Center (SLAC).