A little more salt can prevent batteries from catching fire

Lithium-ion batteries with a new non-flammable electrolyte can continue to operate at high temperatures without igniting.

Rechargeable lithium-ion batteries power phones, laptops, other personal electronics and electric cars, and are even used to store energy produced by solar panels. However, if the temperature of these batteries gets too high, they will stop working and may ignite.

This is partly because the electrolyte inside is flammable, carrying lithium ions between the two electrodes as the battery charges and discharges.

“One of the biggest challenges in the battery industry is this safety issue, so a lot of effort goes into making a safe battery electrolyte,” says Rachel Z Huang, a graduate student at Stanford University and lead author of the book. work at the magazine To be important.

The secret of the new electrolyte? More salt.

Standard battery materials ignite when exposed to flame, but the new material (shown here) does not. (Jian-Cheng Lai/Stanford)

non-combustible electrolytes

Conventional lithium-ion battery electrolytes are made from a lithium salt dissolved in a liquid organic solvent such as ether or carbonate. While this solvent improves battery performance by helping lithium ions move around, it is also a potential fire starter.

Batteries generate heat when operating. And if a battery has holes or defects, it will heat up quickly. At temperatures above 140 degrees F, tiny solvent molecules in the electrolyte begin to evaporate, turning from liquid to gas and inflating a battery like a balloon – until the gas ignites and everything bursts into flames.

In the last 30 years, researchers have developed non-flammable electrolytes, such as polymer electrolytes, which use a polymer matrix instead of a conventional salt-solvent solution to move ions. However, these safer alternatives cannot move ions as efficiently as liquid solvents, so their performance has not been as measured as that of conventional electrolytes.

The team wanted to produce a polymer-based electrolyte that could offer both safety and performance. And Huang had an idea.

He decided to add as much of a lithium salt called LiFSI as he could to a polymer-based electrolyte designed and synthesized by Jian-Cheng Lai, a postdoctoral fellow at Stanford University and co-author of the paper.

“I just wanted to see how much I could add and test the limit,” Huang says. Usually less than 50% by weight of a polymer-based electrolyte is salt. Huang has increased 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 common electrolyte known as Solvent-Linked Non-Flammable Electrolyte (SAFE) has proven to be non-flammable at high temperatures during tests on a lithium-ion battery.

SAFE works because solvents and salt work together. Solvent molecules aid in the conduction of ions, providing performance comparable to batteries containing conventional electrolytes. However, instead of failing at high temperatures like most lithium-ion batteries, batteries with SAFE continue to operate at temperatures between 77–212 degrees F.

Meanwhile, the abundantly added salts act as anchors for the solvent molecules, preventing them from evaporating and igniting.

“This new finding points to a new way of thinking for polymer-based electrolyte design,” says Zhenan Bao, a professor at Stanford University and a researcher at 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.”

sticky electrolytes

Polymer-based electrolytes can be solid or liquid. More importantly, the solvents and salt in SAFE plasticize the polymer matrix to make a viscous liquid, just like conventional electrolytes.

One advantage: A sticky electrolyte can fit into existing commercially available lithium-ion battery parts, unlike other non-flammable electrolytes that come up. Solid-state ceramic electrolytes, for example, have to use specially designed electrodes, making them costly to manufacture.

“With SAFE, there is no need to change the production setup,” Huang says. “Of course, there are optimizations needed to fit the electrolyte into the production line if used for production, but the work is much less than with other systems.”

“This very exciting new battery electrolyte is compatible with existing lithium-ion battery cell technology and will have major implications for consumer electronics and electric transportation,” says Yi Cui, a professor at SLAC and Stanford and a SIMES researcher who is also an advisor to Huang. .

Next up: Electric cars?

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

In addition to reducing the risk of fire, this means less space used by cooling systems and more space for batteries. More batteries increase the overall energy density, which means the vehicle can go longer between charges.

“So it’s not just a security advantage,” Huang says. “This electrolyte could also allow you to pack a lot more batteries.”

Time will tell if other battery powered products can be a little SAFE.

DOE’s Battery Materials Research Program and the Office of Energy Efficiency and Renewable Energy under the Battery 500 Consortium funded the study.

Source: Chris Patrick of Stanford University

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