Electric cars are being artificially bolstered by government mandates and subsidies and are doing little to reduce emissions because the electricity they need is overwhelmingly not solar, nuclear, or hydroelectric.

What would help are batteries that aren't stuck in the 20th century, like lithium-ion, which cost so much to replace that one Tesla owner blew up his car with dynamite rather than a cost for new batteries that was 50 percent of the original purchase price. And they can be dangerous.

Lithium-ion explosion risk

Solid-state batteries, like lithium-ion, create energy as lithium ions move from one side of the battery to the other and electrons flow through a circuit to power the device. A lithium-ion battery helps the lithium ions move quickly using a liquid electrolyte but current liquid electrolytes are flammable and can cause a battery explosion or fire, especially when the battery is damaged.  Solid-state batteries solve a big performance problem but a new study shows there are still obstacles, like a dangerous amount of heat if they short-circuit.

In a solid-state battery, the liquid electrolyte is replaced by a solid electrolyte but 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.  

To overcome that, researchers added liquid electrolyte and found that despite concern they would make also solid-state batteries less safe, solid-state batteries with liquid electrolyte were safer than lithium-ion counterparts. The problem is that the improved performance and higher energy density, a good thing in a battery, means if that extra stored energy shorts the all-solid-state battery could put out a dangerous amount of heat.

Optimizing for safety

In order to figure out just how safe a solid-state battery 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 solid-state batteries with varying amounts of liquid electrolyte. All batteries tested had equivalent amounts of stored energy. Then, they looked at three different bad things that could happen to the batteries, and the heat that would be released due to each type of failure.

The first bad thing that could happen is if the batteries caught on fire — from either a neighboring battery or a surrounding building — Torres-Castro said. In these cases, the researchers found that the solid-state battery 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 solid-state battery without liquid electrolyte didn’t produce any heat under this scenario.

The second bad thing that could happen to the batteries is if repeated charging and discharging caused 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 causes a short-circuit, Preger said. 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.

The third bad thing that could happen to a solid-state battery is the solid electrolyte could break. This could happen if the battery was crushed or punctured or due to built-up pressure during operation, which would allow oxygen from one side of the battery to react with the lithium metal on the other side, Torres-Castro said. In these cases, the solid-state battery without liquid electrolyte could reach temperatures near that of the lithium-ion battery, which the team found surprising.

But the risk is not high. 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.