Researchers at MIT have discovered a way to stop dendrites crossing electrolyte. In a paper published in the journal Joule and reported on MIT’s website, MIT Professor Yet-Ming Chiang said in the group’s earlier work, they made a “surprising and unexpected” finding.
This was that lithium, a soft metal, can penetrate the hard, solid electrolyte material used for a solid-state battery during charging and discharging, as ions of lithium move between the two sides.
That movement causes the volume of the electrodes to change. They found that inevitably causes stresses in the solid electrolyte.
“To deposit this metal, there has to be an expansion of the volume because you’re adding new mass,” Chiang said. “So, there’s an increase in volume on the side of the cell where the lithium is being deposited. And if there are even microscopic flaws present, this will generate a pressure on those flaws that can cause cracking.”
They found those stresses cause the cracks that allow dendrite formation. The solution to the problem is more stress.
Team member and graduate student Cole Fincher developed a way of making thin cells using a transparent electrolyte. This allowed the whole process to be directly seen and recorded.
“You can see what happens when you put a compression on the system, and you can see whether or not the dendrites behave in a way that’s commensurate with a corrosion process or a fracture process,” he says.
The team demonstrated that they could directly manipulate the growth of dendrites by applying and releasing pressure, causing the dendrites to zig zag in perfect alignment with the direction of the force.
They showed that stack pressure makes dendrite formation worse. Pressure along the plane of the plates forces the dendrites to travel in the direction of the compression and not reach other plates.
Chiang says they will now try to create a functional prototype battery. They have filed for a patent, but the researchers do not plan to commercialise the system themselves.
Image courtesy of MIT/the researchers: applying a compression force across a solid electrolyte material (grey disk) caused the dendrite (dark line on left) to stop moving from one electrode toward the other (the round metallic patches at each side) and instead veer harmlessly sideways.