lithium ion batteries

Solid-state batteries could reach commercialisation with liquid electrolyte

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The fabled solid-state battery (SSB) has taken one step closer to reality with a new project that uses a liquid electrolyte to improve performance.

SSBs are typically made with technology that uses solid electrodes and a solid electrolyte, instead of the liquid or polymer gel electrolytes found in lithium-ion or lithium polymer batteries.

While the batteries can already be produced, their low energy density has so far dissuaded manufacturers from embracing the technology.

But a new study led by Sandia National Laboratories in the US is tackling the long-held assumption that adding some liquid electrolyte to improve performance would make solid-state batteries unsafe.

Instead, the research team found that in many cases SSBs with a little liquid electrolyte were safer than their lithium-ion counterparts (pictured). They also found that if the battery were to short-circuit, releasing all its stored energy, the theoretically super-safe, all-solid-state battery could put out a dangerous amount of heat.

“Solid-state batteries have the potential to be safer, and they have the potential for higher energy density,” said Alex Bates, a Sandia postdoctoral researcher who led the study.

“This means, for electric vehicles, you could go farther in between charges, or need fewer batteries for grid-scale energy storage. The addition of liquid electrolyte may help bridge the gap to commercialisation, without sacrificing safety.”

SSBs are somewhat like lithium-ion batteries in that lithium ions move from one side of the battery to the other, while electrons flow through a circuit to power the device. But the main difference is that throughout a lithium-ion battery, there is a substance that helps the lithium ions move quickly, which is the liquid electrolyte.

The electrolyte effectively shuttles lithium ions directly where they need to go, but current liquid electrolytes are flammable and can cause a battery explosion or fire, especially when the battery is damaged.

In an SSB, the liquid electrolyte is replaced by a solid electrolyte that also helps the lithium ions move quickly. One technical challenge is that while the lithium ions can move quickly within the solid electrolyte, they have a hard time moving from the solid electrolyte to the electrodes and vice versa.

Adding a little bit of liquid electrolyte has been shown to speed up the shuttling of electrons from one to the other and thereby improve battery charging speeds and performance.

In order to figure out just how safe an SSB with a little liquid electrolyte would be, the research team started by calculating how much heat could be released in a lithium-ion battery, an all-solid-state battery and SSBs with varying amounts of liquid electrolyte. Three batteries were tested that had equivalent amounts of stored energy and it was then determined how the heat would be released in the event of failure.

In the event of a fire, they found that the SSB with a little liquid electrolyte in it produced about one-fifth of the heat of a comparable lithium-ion battery — depending on how much liquid electrolyte it had. The SSB without liquid electrolyte didn’t produce any heat under this scenario.

They also looked at what could happen to the batteries during repeated charging and discharging that can cause the lithium metal to form a ‘spike’ called a dendrite. This dendrite can puncture a hole through the separator that keeps the two sides distinct and cause a short-circuit. This is a known issue with all batteries that have lithium metal on one side. In this case, all three batteries produced similar amounts of heat, which depended on how much lithium metal was in the batteries.

Finally, the researchers looked at what could happen if the solid electrolyte were to break, perhaps because the battery was crushed or punctured or due to built-up pressure during operation. A breakage could allow oxygen from one side of the battery to react with the lithium metal on the other side. In these cases, the SSB without liquid electrolyte could reach temperatures near that of the lithium-ion battery, which the team found surprising.

“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,” said researcher Yuliya Preger. “This study highlighted the importance of engineering the heck out of that separator so that it does not fail.”

The next steps for the project include conducting similar calculations with other solid electrolyte materials and conducting experiments to validate the new and original calculations.

“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,” Bates added. “What we were trying to point out in this paper is that there’s a definite trade-off between performance and safety, but adding a bit of liquid may greatly increase performance while only having a small impact on safety.”

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