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How smartwatches and pacemakers key to electric car revolution

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Alex Bates and John Hewson, Sandia National Laboratories engineers, examine a lithium-ion battery. (Sandia National Laboratories via SWNS)

By Mark Waghorn via SWNS

Solid-state batteries used in smartwatches and pacemakers hold the key to the electric car revolution, according to new research.

They are potentially safer and more powerful than lithium-ion alternatives.

The devices could also be used for storing energy from solar panels for later use.

Lead author Dr. Alex Bates said: "Solid-state batteries have the potential to be safer and they have the potential for higher energy density.

"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 commercialization, without sacrificing safety."

Lithium-ion batteries use liquid electrolytes and have separators that keep the positive and negative electrodes apart.

Solid-state batteries use thin layers of solid electrolytes - which carry lithium ions between electrodes.

The team at Sandia National Laboratories in New Mexico compared the heat released by a traditional lithium-ion or solid-state battery.

They found in many cases the latter was better - with a little liquid electrolyte added.

The long-held assumption had been improving performance in this way would make them unsafe.

The study in the journal Joule also found if the battery were to short-circuit - releasing all its stored energy - it could put out a dangerous amount of heat.

Mass-market production for electric vehicles is three to five years away, experts say.

They will help speed up a switch from petrol vehicles because drivers would not need to stop as often to charge.

In lithium-ion batteries, the electrolyte is volatile and flammable at high temperatures - which increases the risk of fires and chemical leaks.

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Solid-state batteries are somewhat similar. In both, lithium ions move from one side to the other, while electrons flow through a circuit to power the device.

One big difference is the liquid electrolyte in a lithium-ion battery helps the ions move quickly - instead of a solid electrolyte.

Co-author Dr. Yuliya Preger said: "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.'

"Some scientists say 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.'"

All batteries tested - lithium-ion, solid-state and one with liquid electrolyte - had equivalent amounts of stored energy.

Co-author Dr. John Hewson said: "We started by determining just how much chemical energy is in the three kinds of batteries.

"There's only so much energy you can release, which will heat up the battery a certain amount, if a chemical reaction does occur."

The first bad thing that could happen is if the batteries caught on fire — from either a neighboring battery or a surrounding building.

The solid-state battery with a little liquid electrolyte produced about one-fifth of the heat of a comparable lithium-ion battery.

The second bad thing that could happen is if repeated charging and discharging caused the lithium metal to form a 'spike' called a dendrite.

This can puncture a hole through the separator that keeps the two sides distinct and causes a short-circuit.

In this case, all three batteries produced similar amounts of heat, which depended on how much lithium metal was in the batteries.

Thirdly, the solid electrolyte could break if the battery was crushed or punctured or due to built-up pressure during operation.

This would allow oxygen from one side of the battery to react with the lithium metal on the other side.

In these cases, the battery without liquid electrolyte could reach temperatures near that of the lithium-ion battery.

Dr. Preger said: "One of the promises of solid-state batteries is 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.

"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.

Dr. Bates said: "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.

"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."

Understanding the trade-off may help speed up commercialization. Toyota and Volkswagen plan to mass-produce them by the mid-2020s.

Co-author Dr. Loraine Torres-Castro said: "Having the clarity and the confidence that knowing a small amount of liquid electrolyte will not create huge safety issues may help the development of commercial solid-state batteries.

"Adding liquid electrolyte could fix one of their main problems, the solid electrolyte interface."

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