Solid-state batteries, currently used in small electronic devices like smartwatches, have the potential to be safer and more powerful than lithium-ion batteries for things like electric cars and energy storage. energy from solar panels for later use. However, several technical challenges remain before solid-state batteries become mainstream.
A study led by Sandia National Laboratories, published March 7 in the scientific journal Joule, tackled one such challenge: a long-held assumption that adding liquid electrolyte to improve performance would make solid-state batteries unsafe. Instead, the research team found that in many cases, solid-state batteries with some liquid electrolyte were safer than their lithium-ion counterparts. They also found that if the battery were to short out, releasing all of its stored energy, the theoretically super safe solid-state battery could emit 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 for the item. “This means that, for electric vehicles, you can go further between charges or need fewer batteries for grid-scale energy storage. Adding liquid electrolyte can help bridge the gap. gap with commercialization, without sacrificing safety.”
Better batteries through chemistry
Solid-state batteries look a bit like lithium-ion batteries. In either case, lithium ions move from side to side of the battery, while electrons flow through a circuit to power the device. A big difference is that in a lithium-ion battery there is a substance that helps the lithium ions move quickly: the liquid electrolyte.
Loraine Torres-Castro, a battery safety expert at Sandia’s battery abuse test lab, who is involved in the project, compares the liquid electrolyte to a fleet of cars driving down the aisles: it carries the ions lithium directly where they need to go. However, current liquid electrolytes are flammable and can cause battery explosion or fire, especially when the battery is damaged.
In a solid-state battery, the liquid electrolyte is replaced by a solid material, called a solid electrolyte, which also helps lithium ions move quickly. A technical challenge is that while lithium ions can move quickly through the solid electrolyte, they have difficulty getting from the solid electrolyte to the electrodes and vice versa, Bates said. The solid electrolyte could be compared to a frame of trains, also delivering lithium ions quickly to the station, but passengers still have to travel a little further to get home.
In particular, the scientists accelerated this “direct shuttle” – and therefore the battery’s charging speeds and performance – by adding a little liquid electrolyte to the positive side of the battery.
However, Yuliya Preger, Sandia battery reliability expert on the project, said, “There has been much controversy in the solid-state battery research community regarding the safety of including liquid electrolyte for “grease the wheels”. Some scientists say that any amount of liquid electrolyte is dangerous. So we did the math to see what the impacts of liquid electrolyte might be, instead of just accepting the ‘party line’.
Steve Harris, a battery scientist at Lawrence Berkeley National Laboratory, and Katie Harrison, a battery scientist at Sandia, initially questioned the “party line” that led to the study. Both participated in the study.
How safe are solid-state batteries?
In order to determine how safe a solid-state battery with some liquid electrolyte would be, the research team began by calculating the amount of heat that could be released in a lithium-ion battery, an all-electric battery. semiconductors and a solid-state battery. batteries with varying amounts of liquid electrolyte. All batteries tested had equivalent amounts of stored energy. Next, they looked at three different bad things that could happen to batteries, and the heat that would be released due to each type of failure.
“We started by determining the amount of chemical energy contained in the three battery types,” said John Hewson, a Sandia heat release calculation expert on the project. “There’s only so much energy you can release, which will heat the battery by a certain amount, if a chemical reaction occurs.”
The first bad thing that could happen is if the batteries caught fire — either from a nearby battery or a surrounding building — Torres-Castro said. In these cases, the researchers found that the solid-state battery with some liquid electrolyte produced about one-fifth the heat of a comparable lithium-ion battery, depending on how much liquid electrolyte it had. . The solid-state battery without liquid electrolyte produced no heat in this scenario.
The second bad thing that could happen to batteries is if repeated charging and discharging causes the lithium metal to form a “spike” called a dendrite. This dendrite can punch a hole through the separator that keeps the two sides apart 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, the three batteries produced similar amounts of heat, which depended on the amount of lithium metal contained in the batteries.
The third bad thing that could happen to a solid state battery is that the solid electrolyte could break down. This could happen if the battery was crushed or punctured or due to a pressure build up during operation, which would allow oxygen on one side of the battery to react with metallic lithium on the other side, said Torres-Castro. In these cases, the solid-state battery without liquid electrolyte could reach temperatures close to that of the lithium-ion battery, which the team found surprising.
From safety calculations to laboratory experiments
“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 breaks, the temperature rise could be about as important than when lithium-ion batteries fail,” Preger said. noted. “This study highlighted the importance of designing the heck out of this separator so that it does not fail.”
Next steps for the project include performing similar calculations with other solid electrolyte materials and performing experiments to validate the new and original calculations, Bates said.
“We found that if the solid-state battery contains lithium metal, it can be dangerous whether or not it contains a liquid electrolyte,” he said. “What we’re trying to point out in this article is that there is a definite trade-off between performance and safety, but adding a little liquid can dramatically increase performance with little impact on safety. .”
Understanding this trade-off can help speed time to market, Torres-Castro added. “Having the clarity and confidence that knowing a small amount of liquid electrolyte will not create huge safety issues can help the development of commercial solid-state batteries. Adding liquid electrolyte could solve one of their main problems, the solid electrolyte interface.”
This safety study was supported by the Department of Energy Office’s Electrical Energy Storage Program.