Lithium-ion versus solid state: reassessing battery safety

Solid-state batteries, the new kid on the battery block, have drawn a lot of interest from all corners of the energy industry. Having no flammable liquid electrolyte (hence “solid-state”), they skip many of the safety issues that traditional lithium-ion batteries have, without sacrificing power.

But they’re still in the realm of research, and a new study has questioned the idea that they’d be inherently safer. A paper in Joule has compared solid-state batteries with their more commercialised lithium-ion counterparts, and found mixed results.

Solid-state and lithium-ion batteries both contain lithium (Li): in both, the Li+ ions move from one part of the battery to another, allowing negatively charged electrons to move through a circuit. The key difference is what the ions move through. In a traditional lithium-ion battery, it’s a liquid electrolyte, while a solid-state battery would use a solid material. They’re technically more demanding to make, but they could be more efficient than normal lithium batteries.

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“Solid-state batteries have the potential to be safer, and they have the potential for higher energy density,” says Dr Alex Bates, a researcher at Sandia National Laboratories, US, and lead author on the paper.

“This means, for electric vehicles, you could go farther in between charges, or need fewer batteries for grid-scale energy storage.”

But in their analysis, the researchers found that a small amount of liquid may actually improve the performance of solid-state batteries, without compromising safety overmuch.

“The addition of liquid electrolyte may help bridge the gap to commercialisation, without sacrificing safety,” says Bates.

“There has been a lot of controversy in the solid-state battery research community about the safety of including liquid electrolyte to ‘grease the wheels’,” says co-author Dr Yuliya Preger.

“Some scientists say that any amount of liquid electrolyte is unsafe. So, we did the calculations to see what the impacts of liquid electrolyte could be, instead of just accepting the ‘party line’.”

The researchers tested three different types of battery: a solid-state battery, a lithium-ion battery, and a hybrid that had varying amounts of liquid electrolyte.

The (violent) ways batteries can fail. Credit: Sandia National Laboratories.

They examined three potential failure scenarios for each type of battery: external heating (read: fire), an internal short circuit, and mechanical failure (read: crushed or punctured battery).

In the fire scenario, the battery with a small amount of liquid released about a fifth as much heat as a lithium-ion battery, and the solid-state battery produced no heat. All three batteries performed similarly in the internal short circuit scenario.

Two men in masks in a lab loading a hand-sized battery into a spherical chamber
Alex Bates (front) and John Hewson setting up a battery testing chamber. Credit: Photo by Rebecca Gustaf/Sandia National Laboratories

But in the mechanical failure scenario, the pure solid-state battery could release even more heat than its traditional counterpart.

“One of the promises of solid-state batteries is that they are safe because the solid electrolyte is firm and unlikely to break. But if it does break, the temperature rise could be about as much as when lithium-ion batteries fail,” says Preger.

“This study highlighted the importance of engineering the heck out of that separator so that it does not fail,” she adds.

Bates says they need to investigate other solid electrolyte materials to confirm these findings.

“We found if the solid-state battery has lithium metal, it has the potential to be dangerous, regardless of if it has liquid electrolyte or not,” he says.

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